Making the actionmailer docs more explicit to understand [ci skip]

1. Updated the comments in actionmailer/lib/action_mailer/base.rb to make it more easy to understand while reading them.

2. Added the description to ActionMailer::DeliveryJob class in actionmailer/lib/action_mailer/delivery_job.rb . Even though it is not exposed to public api, the comment should help to better understand the role of the class and when it is of use.
Yes, that is more grammatically correct. And I shall update that too.