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.
What about `It accepts a headers hash.`. 

I think we don't need to specify that it accepts it in both ways of calling.