Update the Mail Handler documentation on what is considered a bulk message

Problem Definition

Thanks to the fix fo JRASERVER-38662, the following conditions that incoming mails are considered as "bulk" have been covered in the corresponding section in the latest document for now.

• The first 'Precedence' header in the message is 'bulk'
• The message contains the header "Auto-Submitted" with a value different than "no"

However, the following conditions still aren't mentioned.

• The message is a "multipart/report" message and the content type contains 'report-type=delivery-status'
• The message is not a "multipart/report" message and the "Return-Path" is "<>"

Suggested Solution

It would be better to mention the conditions in the document as well.