Users: Clarify the return value of `get_current_user_id()` for non-logged-in users.

Props ovenall
Fixes #39051

Built from https://develop.svn.wordpress.org/trunk@39486


git-svn-id: https://core.svn.wordpress.org/trunk@39426 1a063a9b-81f0-0310-95a4-ce76da25c4cd