Document enabling error reporting in devel docs.

Documentation.html

@@ -4457,6 +4457,9 @@ now synchronize your databases placed on the same server or some remote server.
         <abbr title="PHP Extension and Application Repository">PEAR</abbr>
         coding standards</a>
         on the other hand. Please pay attention to this.</li>
+    <li>Please enable showing PHP errors and warnings by the
+        <code><a href="#cfg_Error_Handler_display">$cfg['Error_Handler']['display']</a></code>
+        configuration directive.</li>
     <li>Please try to keep up the file-naming conventions. Table-related stuff
         goes to <i>tbl_*.php</i>, db-related code to <i>db_*.php</i>,
         server-related tools to <i>server_*.php</i> and so on.</li>