Document primary_blog meta key for users

Author: danielbachhuber

src/User_Meta_Command.php

@@ -131,6 +131,10 @@ public function list_( $args, $assoc_args ) {
 	 *     # Get user meta
 	 *     $ wp user meta get 123 bio
 	 *     Mary is an WordPress developer.
+	 *
+	 *     # Get the primary site of a user (for multisite)
+	 *     $ wp user meta get 2 primary_blog
+	 *     3
 	 */
 	public function get( $args, $assoc_args ) {
 		$args = $this->replace_login_with_user_id( $args );