Add suggestions for enqueuing many jobs and emails

[fatkodima]

Discussion - https://github.com/rubocop/rubocop-rails/issues/1517

[Earlopain]

Does this usually say which rails version introduced this? deliver_all_later will be part of 8.1 which is not released yet

[bbatsov]

I guess it's not a bad idea to add an admonition (e.g. NOTE:) to make this clearer.

[fatkodima]

Pushed a note.

When implementing a cop, is it ok to create a single cop for both cases (would be a cop that detects single operations in a loop which can be performed in bulk)? I also want it to be extensible to extend with Sidekiq'q perform_bulk method.