Bug 312089 - document Delete key as Back/Shift+Delete as Forward on Mac. r=steffen

browser/locales/en-US/chrome/help/shortcuts.xhtml

@@ -58,7 +58,8 @@ Contributors:
         </span>
         <span class="mac">
           &accelKey;+<kbd>[</kbd><br/>
-          &accelKey;+<kbd>Left Arrow</kbd>
+          &accelKey;+<kbd>Left Arrow</kbd><br/>
+          <kbd>Delete</kbd>
         </span>
       </td>
     </tr>
@@ -71,12 +72,13 @@ Contributors:
         </span>
         <span class="unix">
           &altKey;+<kbd>Right Arrow</kbd><br/>
-          &shiftKey;+<kbd>Backspace</kbd>
+          &shiftKey;+<kbd>Backspace</kbd><br/>
           &accelKey;+<kbd>]</kbd>
         </span>
         <span class="mac">
           &accelKey;+<kbd>]</kbd><br/>
-          &accelKey;+<kbd>Right Arrow</kbd>
+          &accelKey;+<kbd>Right Arrow</kbd><br/>
+          &shiftKey;+<kbd>Delete</kbd>
         </span>
       </td>
     </tr>