Docs: Document the usage of $wpdb global in wp_get_post_autosave().

Follow-up to [48422].

Props ravivaddweb.
Fixes #51308. See #34560.

git-svn-id: https://develop.svn.wordpress.org/trunk@48975 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/revision.php

@@ -227,6 +227,8 @@ function wp_save_post_revision( $post_id ) {
  *
  * @since 2.6.0
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param int $post_id The post ID.
  * @param int $user_id Optional The post author ID.
  * @return WP_Post|false The autosaved data or false on failure or when no autosave exists.