doc: security fixes are prepared for two most recent stable releases

Change-Id: Ia22f681b0e90876ec87363c50c6a284db050f649
Reviewed-on: https://go-review.googlesource.com/19793Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/security.html

@@ -71,7 +71,7 @@ This person coordinates the fix and release process.</li>
 <li>If it is determined, in consultation with the submitter, that a CVE-ID is
 required, the primary handler obtains one via email to
 <a href="http://oss-security.openwall.org/wiki/mailing-lists/distros">oss-distros</a>.</li>
-<li>Fixes are prepared for the current stable release and the head/master
+<li>Fixes are prepared for the two most recent major releases and the head/master
 revision. These fixes are not yet committed to the public repository.</li>
 <li>A notification is sent to the
 <a href="https://groups.google.com/group/golang-announce">golang-announce</a>