doc/1.9: add mention of net/http.LocalAddrContextKey

Fixes #21603

Change-Id: I42fb7ea2dd7f6d6a201171055beaeda68c26b823
Reviewed-on: https://go-review.googlesource.com/59530
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tombergan

doc/go1.9.html

@@ -745,6 +745,11 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
         and
         <a href="/pkg/context/#WithValue"><code>context.WithValue</code></a> instead.
       </li>
+
+      <li><!-- CL 35490 -->
+        <a href="/pkg/net/http/#LocalAddrContextKey"><code>LocalAddrContextKey</code></a> now contains
+        the connection's actual network address instead of the interface address used by the listener.
+      </li>
     </ul>
 
     <p>Client &amp; Transport changes:</p>