doc: document new ParseMultipartForm behavior

Fixes golang#8403.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/123860043

doc/go1.3.html

@@ -521,6 +521,15 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 client.
 </li>
 
+<li>
+The <a href="/pkg/net/http/"><code>net/http</code></a> package's
+<a href="/pkg/net/http/#Request.ParseMultipartForm"><code>Request.ParseMultipartForm</code></a>
+method will now return an error if the body's <code>Content-Type</code>
+is not <code>mutipart/form-data</code>.
+Prior to Go 1.3 it would silently fail and return <code>nil</code>.
+Code that relies on the previous behavior should be updated.
+</li>
+
 <li> In the <a href="/pkg/net/"><code>net</code></a> package,
 the <a href="/pkg/net/#Dialer"><code>Dialer</code></a> struct now
 has a <code>KeepAlive</code> option to specify a keep-alive period for the connection.