Docs: Add missing @global reference for the $post_id global in wp_admin_bar_edit_menu().

Follow-up to [47903].

Props davidbaumwald.
See #50312.

git-svn-id: https://develop.svn.wordpress.org/trunk@47906 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/admin-bar.php

@@ -692,6 +692,7 @@ function wp_admin_bar_shortlink_menu( $wp_admin_bar ) {
  * @global WP_Query $wp_the_query WordPress Query object.
  * @global int      $user_id      The ID of the user being edited. Not to be confused with the
  *                                global $user_ID, which contains the ID of the current user.
+ * @global int      $post_id      The ID of the post when editing comments for a single post.
  *
  * @param WP_Admin_Bar $wp_admin_bar
  */