[MINOR][BUILD] Fix javadoc8 break

## What changes were proposed in this pull request?

These error below seems caused by unidoc that does not understand double commented block.

```
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:69: error: class, interface, or enum expected
[error]  * MapGroupsWithStateFunction<String, Integer, Integer, String> mappingFunction =
[error]                                  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:69: error: class, interface, or enum expected
[error]  * MapGroupsWithStateFunction<String, Integer, Integer, String> mappingFunction =
[error]                                                                       ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:70: error: class, interface, or enum expected
[error]  *    new MapGroupsWithStateFunction<String, Integer, Integer, String>() {
[error]                                         ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:70: error: class, interface, or enum expected
[error]  *    new MapGroupsWithStateFunction<String, Integer, Integer, String>() {
[error]                                                                             ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:72: error: illegal character: '#'
[error]  *      @Override
[error]          ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:72: error: class, interface, or enum expected
[error]  *      @Override
[error]              ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:73: error: class, interface, or enum expected
[error]  *      public String call(String key, Iterator<Integer> value, KeyedState<Integer> state) {
[error]                ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:73: error: class, interface, or enum expected
[error]  *      public String call(String key, Iterator<Integer> value, KeyedState<Integer> state) {
[error]                                                    ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:73: error: class, interface, or enum expected
[error]  *      public String call(String key, Iterator<Integer> value, KeyedState<Integer> state) {
[error]                                                                ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:73: error: class, interface, or enum expected
[error]  *      public String call(String key, Iterator<Integer> value, KeyedState<Integer> state) {
[error]                                                                                     ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:73: error: class, interface, or enum expected
[error]  *      public String call(String key, Iterator<Integer> value, KeyedState<Integer> state) {
[error]                                                                                                 ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:76: error: class, interface, or enum expected
[error]  *          boolean shouldRemove = ...; // Decide whether to remove the state
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:77: error: class, interface, or enum expected
[error]  *          if (shouldRemove) {
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:79: error: class, interface, or enum expected
[error]  *          } else {
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:81: error: class, interface, or enum expected
[error]  *            state.update(newState); // Set the new state
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:82: error: class, interface, or enum expected
[error]  *          }
[error]  ^
[error] .../forked/spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:85: error: class, interface, or enum expected
[error]  *          state.update(initialState);
[error]  ^
[error] .../forked/spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:86: error: class, interface, or enum expected
[error]  *        }
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:90: error: class, interface, or enum expected
[error]  * </code></pre>
[error]  ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:92: error: class, interface, or enum expected
[error]  * tparam S User-defined type of the state to be stored for each key. Must be encodable into
[error]            ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:93: error: class, interface, or enum expected
[error]  *           Spark SQL types (see {link Encoder} for more details).
[error]                                          ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:94: error: class, interface, or enum expected
[error]  * since 2.1.1
[error]           ^
```

And another link seems unrecognisable.

```
.../spark/sql/core/target/java/org/apache/spark/sql/KeyedState.java:16: error: reference not found
[error]  * That is, in every batch of the {link streaming.StreamingQuery StreamingQuery},
[error]
```

Note that this PR does not fix the two breaks as below:

```
[error] .../spark/sql/core/target/java/org/apache/spark/sql/DataFrameStatFunctions.java:43: error: unexpected content
[error]    * see {link DataFrameStatsFunctions.approxQuantile(col:Str* approxQuantile} for
[error]      ^
[error] .../spark/sql/core/target/java/org/apache/spark/sql/DataFrameStatFunctions.java:52: error: bad use of '>'
[error]    * param relativeError The relative target precision to achieve (>= 0).
[error]                                                                     ^
[error]
```

because these seem probably fixed soon in apache#16776 and I intended to avoid potential conflicts.

## How was this patch tested?

Manually via `jekyll build`

Author: hyukjinkwon <[email protected]>

Closes apache#16926 from HyukjinKwon/javadoc-break.

Author: HyukjinKwon

sql/core/src/main/scala/org/apache/spark/sql/KeyedState.scala

@@ -17,8 +17,6 @@
 
 package org.apache.spark.sql
 
-import java.lang.IllegalArgumentException
-
 import org.apache.spark.annotation.{Experimental, InterfaceStability}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
 
@@ -36,7 +34,7 @@ import org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
  * `Dataset.groupByKey()`) while maintaining user-defined per-group state between invocations.
  * For a static batch Dataset, the function will be invoked once per group. For a streaming
  * Dataset, the function will be invoked for each group repeatedly in every trigger.
- * That is, in every batch of the [[streaming.StreamingQuery StreamingQuery]],
+ * That is, in every batch of the `streaming.StreamingQuery`,
  * the function will be invoked once for each group that has data in the batch.
  *
  * The function is invoked with following parameters.
@@ -65,7 +63,7 @@ import org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
  *
  * Scala example of using KeyedState in `mapGroupsWithState`:
  * {{{
- * /* A mapping function that maintains an integer state for string keys and returns a string. */
+ * // A mapping function that maintains an integer state for string keys and returns a string.
  * def mappingFunction(key: String, value: Iterator[Int], state: KeyedState[Int]): String = {
  *   // Check if state exists
  *   if (state.exists) {
@@ -88,7 +86,7 @@ import org.apache.spark.sql.catalyst.plans.logical.LogicalKeyedState
  *
  * Java example of using `KeyedState`:
  * {{{
- * /* A mapping function that maintains an integer state for string keys and returns a string. */
+ * // A mapping function that maintains an integer state for string keys and returns a string.
  * MapGroupsWithStateFunction<String, Integer, Integer, String> mappingFunction =
  *    new MapGroupsWithStateFunction<String, Integer, Integer, String>() {
  *