Plugins: Replace usage of $this in action and filter parameter docblocks with more appropriate variable names.

See #51800, #52217

Fixes #52243


git-svn-id: https://develop.svn.wordpress.org/trunk@49946 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp.php

@@ -485,7 +485,7 @@ class WP {
 		 * @since 2.8.0
 		 *
 		 * @param string[] $headers Associative array of headers to be sent.
-		 * @param WP       $this    Current WordPress environment instance.
+		 * @param WP       $wp      Current WordPress environment instance.
 		 */
 		$headers = apply_filters( 'wp_headers', $headers, $this );