Docs: Add null to `post_id` param type, fix syntax, and descriptions in the...

Docs: Add null to `post_id` param type, fix syntax, and descriptions in the DocBlock for `wp_update_comment_count()`.

See #32246.


Built from https://develop.svn.wordpress.org/trunk@36139


git-svn-id: https://core.svn.wordpress.org/trunk@36105 1a063a9b-81f0-0310-95a4-ce76da25c4cd