count_user_posts() - change docs to return string instead of int

[nofearinc]

As seen in a tweet by Rarst and tested locally, count_user_posts returns the number of posts from a user as a string instead of int calling $wpdb->get_var. Since people may validate against strings or compare accordingly, a safe bet is letting IDEs know that the return type is string.

[nofearinc]

Related: #33929

[SergeyBiryukov]

In 36085:

Docs: Correct @return type for count_user_posts().

Props nofearinc.
Fixes #35222.