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

Props upadalavipul.
See #57069, #56792.


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git-svn-id: http://core.svn.wordpress.org/trunk@54429 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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