RATIONALE
Purpose of this commit is to cleanup main post helpers and have a more easy
and understandable way of calling them.
SPECIFICATIONS
Update methods parameters to look more like 'message_post' definition. Add
docstrings to explain parameter usage. Cleanup some parameter we do not
want. Add a api.returns on message_notify to wrap the value in case of external
call, as this method is public, like message_post.
Task-2710804 (Mail: Clean MailThread Posting API)
Part-of: odoo/odoo#99482