ActionMailer test helpers

Hi,

I recently found these two helpers (assert_emails, assert_no_emails) inside ActionMailer gem:

https://github.com/rails/rails/blob/master/actionmailer/lib/action_mailer/test_helper.rb

I wonder why they are not mentioned in the official documentation:

Are they deprecated ? Should I use them or better to stick with assert_difference 'ActionMailer::Base.deliveries.size’, +1 do`` ?

Regards,

Anton