Ensure `$post_id` is documented as optional where applicable.

See #28388.

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Be more specific that `$post_id` does not default to the ID of the global `$post` in some cases.

See [28654]. See #28388.

Built from https://develop.svn.wordpress.org/trunk@28655


git-svn-id: http://core.svn.wordpress.org/trunk@28473 1a063a9b-81f0-0310-95a4-ce76da25c4cd