Docs: Replace $this in hook param docs with more appropriate names.

`$this` is a pseudo-variable that cannot be used as the name of a function parameter, so renaming these helps prevent errors when implementing hook callback functions.

Fixes #53457

Built from https://develop.svn.wordpress.org/trunk@51518


git-svn-id: http://core.svn.wordpress.org/trunk@51129 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-date-query.php

@@ -555,7 +555,7 @@ class WP_Date_Query {
 		 * @since 3.7.0
 		 *
 		 * @param string        $where WHERE clause of the date query.
-		 * @param WP_Date_Query $this  The WP_Date_Query instance.
+		 * @param WP_Date_Query $query The WP_Date_Query instance.
 		 */
 		return apply_filters( 'get_date_sql', $where, $this );
 	}