wp-includes/user.php: PhpDoc correction - $user_id is not a Post ID

[Corphi]

To cite wp-includes/user.php, for functions add_user_meta(), get_user_meta() and update_user_meta():
* @param int $user_id Post ID.

In Version 3.7.1 it’s the lines 815, 852 and 874 respectively. $user_id is obviously not a Post ID, but a User ID.

[SergeyBiryukov]

In 25997:

Correct $user_id parameter description. props mauryaratan, Corphi. fixes #25774.