Add a doc-string explaining the purpose of each call back and
how they should be used.
Also add a currently unused callback 'POST_FAIL' that is to be used in
cases the action fails - giving script authors, a guarantee that a
call to `pre` will always have a matching `post/post_fail` call.
- D11422: adds a callback that can use 'post_fail'.
- T88696: proposed these conventions.
507c19c0f7