Docs: Add missing parameter and return descriptions to the DocBlock for `WP_Theme::__isset()`.

See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@36734 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme.php

@@ -343,7 +343,8 @@ final class WP_Theme implements ArrayAccess {
 	 *
 	 * @staticvar array $properties
 	 *
-	 * @return bool
+	 * @param string $offset Property to check if set.
+	 * @return bool Whether the given property is set.
 	 */
 	public function __isset( $offset ) {
 		static $properties = array(