Incorrect datatype for $post_type param in PHPDocs for count_many_users_posts

[willmot]

count_many_users_posts has supported passing the post_type as a second param since #17220 [17742] however the @param PHPDocs for the $post_type parameter incorrectly list the datatype as string|array when in-fact it should be string.

Passing an array causes a PHP error because the $post_type param is passed to get_posts_by_author_sql which assumes it is a string.

Warning: Illegal offset type in isset or empty in /wp-includes/post.php on line 840

Attached patch sets the PHPDocs @param datatype for the post_type param to string.

In [21747]:

Update phpdoc for count_many_users_posts(). Props willmot. fixes #21640