Bug 314966 Search online/offline option in MailNews needs to be documented

p=giacomo.magnini@portalis.it r=me

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

@@ -2556,6 +2556,12 @@
     folder through which you want to search.</li>
   <li>Select <q>Search subfolders</q> to include all subfolders in the
     search.</li>
+  <li>Select <q>Search local system</q> to search only messages from newsgroups
+    or IMAP accounts that have been saved locally.
+
+    <p><strong>Note</strong>: The checkbox will be disabled if it&apos;s not
+      possible to search remotely stored messages.</p>
+  </li>
   <li>Select which matching option Mail &amp; Newsgroups will use to search for
     messages that match all or at least one of the conditions (criteria) that
     you choose.</li>