Docs: Improve various globals documentation, as per documentation standards.

Props upadalavipul.
See #57069, #56792.



git-svn-id: https://develop.svn.wordpress.org/trunk@54877 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/comment.php

@@ -3783,6 +3783,8 @@ function wp_register_comment_personal_data_eraser( $erasers ) {
  *
  * @since 4.9.6
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @param string $email_address The comment author email address.
  * @param int    $page          Comment page.
  * @return array