Document the link function's error conditions. (#165)

* Document the `link` function's error conditions.

Document that `link` fails if old-path doesn't exist or isn't a file,
or if new-path already exists.

* Autogenerate imports.md.

Author: sunfishcode

imports.md

@@ -1157,6 +1157,9 @@ WASI.</p>
 </ul>
 <h4><a id="method_descriptor_link_at"></a><code>[method]descriptor.link-at: func</code></h4>
 <p>Create a hard link.</p>
+<p>Fails with <a href="#error_code.no_entry"><code>error-code::no-entry</code></a> if the old path does not exist,
+with <a href="#error_code.exist"><code>error-code::exist</code></a> if the new path already exists, and
+<a href="#error_code.not_permitted"><code>error-code::not-permitted</code></a> if the old path is not a file.</p>
 <p>Note: This is similar to <code>linkat</code> in POSIX.</p>
 <h5>Params</h5>
 <ul>

wit/types.wit

@@ -508,6 +508,10 @@ interface types {
 
         /// Create a hard link.
         ///
+        /// Fails with `error-code::no-entry` if the old path does not exist,
+        /// with `error-code::exist` if the new path already exists, and
+        /// `error-code::not-permitted` if the old path is not a file.
+        ///
         /// Note: This is similar to `linkat` in POSIX.
         @since(version = 0.2.0)
         link-at: func(