Add example for deleting multiple post IDs (#387)

The documentation isn't clear that to delete multiple posts the IDs must be space separated, not comma separated

Author: joeldcanfield

src/Post_Command.php

@@ -469,6 +469,12 @@ public function get( $args, $assoc_args ) {
 	 *     $ wp post delete 123 --force
 	 *     Success: Deleted post 123.
 	 *
+	 *     # Delete multiple posts
+	 *     $ wp post delete 123 456 789
+	 *     Success: Trashed post 123.
+	 *     Success: Trashed post 456.
+	 *     Success: Trashed post 789.
+	 *
 	 *     # Delete all pages
 	 *     $ wp post delete $(wp post list --post_type='page' --format=ids)
 	 *     Success: Trashed post 1164.