Code Quality: Document that sanitize_key() returns lowercase-string.

This is a narrower PHPStan type compared to just `string`.

See #64898.


git-svn-id: https://develop.svn.wordpress.org/trunk@62836 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-includes/formatting.php

@@ -2186,6 +2186,7 @@ function sanitize_user( $username, $strict = false ) {
  *
  * @param string $key String key.
  * @return string Sanitized key.
+ * @phpstan-return lowercase-string
  */
 function sanitize_key( $key ) {
 	$sanitized_key = '';