Docs: Fix @var type for WP_User_Request::$user_id to string

[masteradhoc]

Fix the inline documentation for WP_User_Request::$user_id, which is
documented as int but is actually a string. The value is assigned
directly from $post->post_author in the constructor with no type cast,
so it reflects the raw database value, which is always a string.

This incorrect type hint was the root cause of the bug described in the
ticket, where a strict comparison against 0 in get_user_locale()
evaluated incorrectly because "0" !== 0. The runtime fix was addressed
in #43985; this corrects the documentation to match actual behavior.

Precedent: WP_Post::$post_author is documented as string for the same
reason (see #25092).

Trac ticket: https://core.trac.wordpress.org/ticket/44723

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[apermo]

@westonruter your thoughts?

[apermo]

If the phpdoc is updated to string, the actual value is wrong.