doc/go1.15: add release notes for regexp

Updates #37419

Change-Id: I340efe55b9dc41bb9ef6c9f0ec158d58a9445864
Reviewed-on: https://go-review.googlesource.com/c/go/+/236738
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.15.html

@@ -537,7 +537,9 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
   <dd>
     <p><!-- CL 187919 -->
-      TODO: <a href="https://golang.org/cl/187919">https://golang.org/cl/187919</a>: add (*Regexp).SubexpIndex
+      The new <a href="/pkg/regexp/#Regexp.SubexpIndex"><code>Regexp.SubexpIndex</code></a>
+      method returns the index of the first subexpression with the given name
+      within the regular expression.
     </p>
   </dd>
 </dl><!-- regexp -->