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.

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git-svn-id: http://core.svn.wordpress.org/trunk@28427 1a063a9b-81f0-0310-95a4-ce76da25c4cd