doc: mention the new purego implementation of hash/maphash

Change-Id: Ieedddf52ae09cf5a8e9347f0f30aff21e8369787
Reviewed-on: https://go-review.googlesource.com/c/go/+/499281
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Bypass: Keith Randall <[email protected]>

Author: randall77

doc/go1.21.html

@@ -514,12 +514,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
   <dd>
-    <p><!-- https://go.dev/issue/47342 -->
-      TODO: <a href="https://go.dev/issue/47342">https://go.dev/issue/47342</a>: Provide a `purego` implementation
-    </p>
-
-    <p><!-- CL 468795 -->
-      TODO: <a href="https://go.dev/cl/468795">https://go.dev/cl/468795</a>: hash/maphash: add purego implementation
+    <p><!-- https://go.dev/issue/47342, CL 468795 -->
+      The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
     </p>
   </dd>
 </dl><!-- hash/maphash -->