Add missing doc blocks to wp-admin/includes/*.

Fix some egregious uses of tabbing.
Some functions can simply return `apply_filters(...)` instead of setting a variable that is immediately returned.

See #32444.


git-svn-id: https://develop.svn.wordpress.org/trunk@32654 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-users-list-table.php

@@ -56,6 +56,8 @@ class WP_Users_List_Table extends WP_List_Table {
 	 *
  	 * @since 3.1.0
 	 * @access public
+	 *
+	 * @return bool
 	 */
 	public function ajax_user_can() {
 		if ( $this->is_site_users )