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, we could just say "It accepts ...".  Earlier it was mentioned "both methods accept", so I thought its better to mention that it is the 2 ways of calling this main method which accepts .... 

But thinking more about it now, it probably makes more sense to just start with "It ..". Thanks @prathamesh-sonpatki , I will update the PR.