Doc/usage of `@format` should update

[ArcturusZhang]

Clear and concise description of the problem

Per previous discussion with @timotheeguerin @iscai-msft and @tadelesh , @format should not be used nor be recognized by TCGC therefore our generator will not even know the existence of these decorator which is quite misleading for our users.

Checklist

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.