doc/go1.5: mention net/http.Request.Cancel

This was added in commit 8b4278ff.

Change-Id: I95279f2779c2bab2767e34389fb4324900c01e6c
Reviewed-on: https://go-review.googlesource.com/11952Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -1039,6 +1039,13 @@ Also, starting with Go 1.5 the package sets the <code>REMOTE_PORT</code>
 variable.
 </li>
 
+<li>
+The <a href="/pkg/net/http/"><code>net/http</code></a> package's
+<a href="/pkg/net/http#Request"><code>Request</code></a> type adds a
+<code>Cancel</code> field, a channel to signal that the request has been
+canceled.
+</li>
+
 <li>
 The <a href="/pkg/net/mail/"><code>net/mail</code></a> package
 adds a <a href="/pkg/net/mail/#AddressParser"><code>AddressParser</code></a>