Pass `$post->ID` to `get_comments_number` filter.

Also fixes indentation and a typo in corresponding filter docs.

see #26240.
Built from https://develop.svn.wordpress.org/trunk@28604


git-svn-id: http://core.svn.wordpress.org/trunk@28428 1a063a9b-81f0-0310-95a4-ce76da25c4cd