allow preview interceptors to be registered through `config.action_mailer`.

This was partially broken because `preview_interceptors=` just assigned the
raw values, whithout going through `register_preview_interceptor`. Now the
Action Mailer railtie takes care of the `preview_interceptors` option.

This commit is a partial revert of:

Revert "Merge pull request #15739 from y-yagi/correct_doc_for_action_mailer_base"

This reverts commit a15704d7f35f17d34d0118546799141d6f853656, reversing
changes made to 1bd12a8609d275ad75fcc4b622ca4f5b32dc76be.

/cc @kuldeepaggarwal @y-yagi