[release-branch.go1.15] doc/go1.14: document json.Umarshal map key support of TextUnmarshaler

Document that json.Unmarshal supports map keys whose underlying
types implement encoding.TextUnmarshaler.

Updates #38801.
Fixes #41178.

Change-Id: Icb9414e9067517531ba0da910bd4a2bb3daace65
Reviewed-on: https://go-review.googlesource.com/c/go/+/237857
Reviewed-by: Daniel Martí <[email protected]>
Run-TryBot: Daniel Martí <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
(cherry picked from commit 47b4509)
Reviewed-on: https://go-review.googlesource.com/c/go/+/252618
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: ToastCheng

doc/go1.14.html

@@ -609,6 +609,12 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       If a program needs to accept invalid numbers like the empty string,
       consider wrapping the type with <a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>.
     </p>
+
+    <p><!-- CL 200237 -->
+      <a href="/pkg/encoding/json/#Unmarshal"><code>Unmarshal</code></a>
+      can now support map keys with string underlying type which implement
+      <a href="/pkg/encoding/#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>.
+    </p>
   </dd>
 </dl><!-- encoding/json -->