Add more clarifying explanation about OutBlocks

Author: danielyli

mllib/src/main/scala/org/apache/spark/ml/recommendation/ALS.scala

@@ -965,7 +965,15 @@ object ALS extends DefaultParamsReadable[ALS] with Logging {
    *     )
    * }}}
    *
-   * The data structure encodes the following information:
+   * Each value in this map-like sequence is of type `Array[Array[Int]]`.  The values in the
+   * inner array are the ranks of the sorted user IDs in that partition; so in the example above,
+   * `Array(0, 1)` in partition 0 refers to user IDs 0 and 6, since when all unique user IDs in
+   * partition 0 are sorted, 0 is the first ID and 6 is the second.  The position of each inner
+   * array in its enclosing outer array denotes the partition number to which item IDs map; in the
+   * example, the first `Array(0, 1)` is in position 0 of its outer array, denoting item IDs that
+   * map to partition 0.
+   *
+   * In summary, the data structure encodes the following information:
    *
    *   *  There are ratings with user IDs 0 and 6 (encoded in `Array(0, 1)`, where 0 and 1 are the
    *      indices of the user IDs 0 and 6 on partition 0) whose item IDs map to partitions 0 and 1