Add missing documentation for the `$password` parameter, passed to the `check_password` hook.

Props coffee2code.
Fixes #30311.

Built from https://develop.svn.wordpress.org/trunk@30381


git-svn-id: http://core.svn.wordpress.org/trunk@30378 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/pluggable.php

@@ -1948,6 +1948,7 @@ function wp_check_password($password, $hash, $user_id = '') {
 		 * @since 2.5.0
 		 *
 		 * @param bool   $check    Whether the passwords match.
+		 * @param string $password The plaintext password.
 		 * @param string $hash     The hashed password.
 		 * @param int    $user_id  User ID.
 		 */