Documentation update for bug 219620

r=rlk and r=neil, a=chofmann

extensions/help/resources/locale/en-US/mail_help.xhtml

@@ -531,7 +531,8 @@ grouped with all its responses:</p>
 
 <ul>
 <li>Click the thread button to the left of the Subject, Sender, and
-Date column headings.</li>
+Date column headings. Or, open the View menu, choose Sort by, and then
+select Threaded.</li>
 </ul>
 
 <table>
@@ -547,6 +548,11 @@ vspace="0" /></td>
 </tr>
 </table>
 
+<p><strong>Tip</strong>: The thread button automatically sorts the threads
+after the age of their parent messages. If you want to use another
+sort criterion for the threads, open the View menu and select the desired
+option from the Sort by submenu.</p>
+
 <p><strong>Tip</strong>: To help you identify unread messages in a
 collapsed thread where you've read the parent message, &brandShortName; Mail
 &amp; Newsgroups underlines the parent message.</p>
@@ -2764,8 +2770,6 @@ by filtering displayed messages.</p>
 <span class="ui">Messages</span>. Choose an option from the submenu.</p>
 
 <ul>
-  <li><span class="ui">Threaded</span>: Choose this option to view messages
-    organized into threads.</li>
   <li><span class="ui">All</span>: Choose this option to view all messages.</li>
   <li><span class="ui">Unread</span>: Choose this option to view only unread messages.</li>
   <li><var>labels</var>: Choose a <a href="#labeling_messages">label</a>