Fix parameter description for the `$post_id` argument in `get_comments_number()` to note that it is optional.

Also fixes the corresponding filter docs, as `$post_id`, not `$post` is passed to the filter.

See [27156]. Fixes #26240.


git-svn-id: https://develop.svn.wordpress.org/trunk@28602 602fd350-edb4-49c9-b593-d223f7449a82