document Intercepters in ActionMailer guide

I refere people a lot to the Rails guides when they are looking for an overview about a specific part of rails. While referring someone to the Action Mailer guide I noticed that there is no mention of the mail interceptors. I think this is quite a valuable feature so I added a small section to explain the basics.

I wanted to add a real-life example but it uses a custom rails env called "staging". Do you think this is confusing?