doc: document that weekly.html is only a historical reference

Fixes #4810.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7401050

doc/devel/weekly.html

@@ -3,18 +3,9 @@
 }-->
 
 <p>This page summarizes the changes between tagged weekly snapshots of Go.
-For full details, see the <a href="http://code.google.com/p/go/source/list">Mercurial change log</a>.</p>
+Such snapshots are no longer created. This page remains as a historical reference only.</p>
 
-<p>Weekly snapshots occur often and may not be stable.
-If stability of API and code is more important than having the
-latest features, use the <a href="release.html">official releases</a> instead.</p>
-
-<p>To update to a specific snapshot, use:</p>
-
-<pre>
-hg pull
-hg update weekly.<i>YYYY-MM-DD</i>
-</pre>
+<p>For recent information, see the <a href="http://code.google.com/p/go/source/list">Mercurial change log</a> and <a href="http://groups.google.com/group/golang-dev/">development mailing list</a>.</p>
 
 <h2 id="2012-03-27">2012-03-27 (<a href="release.html#go1">Go 1</a>)</h2>