Thread:Project talk:Support desk/Notification in the Edit Section/reply

The message that is displayed while editing MediaWiki messages (such as MediaWiki:Common.css) is MediaWiki:Editinginterface.

You can create the "MediaWiki:Editnotice-N" message in your wiki to show a notice to users editing pages in the "N" namespace number (MediaWiki:Editnotice-0 for articles, MediaWiki:Editnotice-3 for users' talk pages, etc.)

See Help:Edit notice for further documentation.