REST API: Always fire the `rest_insert_*` actions after the related object is updated or inserted.

Brings consistency to the `rest_insert_*` actions. Also includes some shuffling and clean-up as well including:
- Ensure we are passing the most current `$post` and `$user` objects to the `update_additional_fields_for_object()` callbacks.
- Changes the function signature of `handle_status_param()` in the Comments controller to accept just the comment_id as the 2nd parameter, instead of a full WP_Comment object. Only the comment_id is needed in the method, this avoids having to include another `get_comment()` call. 
- Renames a variable in the `create_item()` method of the Posts controller from `$post` -> `$prepared_post` to be more explicit.
- Minor fixes/clarifications to the rest_insert_* hook docs

Props rachelbaker, joehoyle
Fixes #38905.
Built from https://develop.svn.wordpress.org/trunk@39348


git-svn-id: https://core.svn.wordpress.org/trunk@39288 1a063a9b-81f0-0310-95a4-ce76da25c4cd