N&N for method to specify message strings as known to be used

- see eclipse-jdt/eclipse.jdt.ui#562

4.28/jdt.html

@@ -130,6 +130,14 @@ <h2>Debug</h2>
     <h2>JDT Developers</h2>
     </td>
   </tr>
+  <tr id="new-method-to-mark-used-message keys">
+  <td class="title"><a href="#new-method-to-mark-used-message-keys">Method to mark message keys used</a></td>
+  <td class="content">
+      When developing Java code, there are set mechanisms for handling message translation.  There is also a context action <b>Source &gt; Find Broken Externalized Strings</b> which checks <i>.properties</i> file and Java message translation classes that access the <i>.properties</i> file.  The action checks for a number of issues including whether a message key is unused or missing and provides the results in the <b>Search</b> dialog.
+   <p>When detecting unused keys, it is possible for the action to generate false positives.  For example, a message could be used by another plug-in or product which is not in the workspace or a message key could be dynamically created and the action cannot detect this.</p>
+   <p>A new mechanism has been added for a developer to denote that a message key is known to be used and should be ignored by the <b>Find Broken Externalized Strings</b> action.  Simply copy any known used messages from the existing <i>.properties</i> file into a <i>.usedproperties</i> file with the same name, in the same package.  The <i>.usedproperties</i> file follows the same syntax as a <i>.properties</i> file and any message keys added there will not be marked as unused by the action.  The message data in the <i>.usedproperties</i> file is ignored and can be removed if desired.</p>
+   </td>
+  </tr>
   <!-- *********************** End of JDT Developers ******************************** -->
   <tr><td colspan="2"/></tr>
 </tbody>