Network admin multisite checks and docs. see #14435.

git-svn-id: https://develop.svn.wordpress.org/trunk@16272 602fd350-edb4-49c9-b593-d223f7449a82

wp-admin/network/plugins.php

@@ -3,10 +3,14 @@
  * Network Plugins administration panel.
  *
  * @package WordPress
- * @subpackage Administration
+ * @subpackage Multisite
  * @since 3.1.0
  */
 
+/** Load WordPress Administration Bootstrap */
 require_once( './admin.php' );
 
+if ( ! is_multisite() )
+	wp_die( __( 'Multisite support is not enabled.' ) );
+
 require( '../plugins.php' );