add keyboard shortcuts for Mac

Author: Maxhodges

msgfmt:ui/lib/ui.html

@@ -113,7 +113,9 @@ <h2>{{mf 'mf_site_translations' 'Site Translations'}}</h2>
         Case insensitive ordering
       </label>
       <br />
-      Use ctrl-up and ctrl-down to quickly change keys & click on headers to reorder the columns.
+      Keyboard shortcuts: Ctrl-up/down arrow (Win); Control-Command-up/down arrow (Mac)
+      <br />
+      Click on headers to reorder the columns.
     </p>
     <span><b>{{keyInfo.key}}</b> in {{keyInfo.file}}:{{keyInfo.line
       }}{{#if keyInfo.template}} (template