Update Mail Handler Documentation

After did some further investigation in https://support.atlassian.com/browse/JSP-193355 support request, it seem our Create a new issue or add a comment to an existing issue article information is not complete.

Perhaps, we can include the following details:

An email is classified as 'bulk' if any of the following is true:

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

If an email is 'bulk', there are three possible actions:

• The mail is ignored, the debug log should have the line "Ignoring email with bulk delivery type", adds the log line 'Rejecting message due to failed bulk check.' and the issue/comment is not created.
• The mail is forwarded, the debug log should have the line "Forwarding email with bulk delivery type", adds the log line 'Rejecting message due to failed bulk check.' and the issue/comment is not created.
• The mail is deleted, the debug log should have the line "Deleting email with bulk delivery type", adds the log line 'Rejecting message due to failed bulk check.' and the issue/comment is not created.

If the email is 'bulk' but the MailHandler does not have a valid config for 'bulk=', it will be processed as any regular (i.e. non bulk) mail. Note: this assumption needs to be tested, the MailHandler might set a default 'bulk' value)

We think the doc need to add more information, as in it does not include the "multipart/report" cases, it does not mention that not configuring "bulk" will include those emails in the regular process (see note above) and does not makes it clear enough that configuring 'bulk' means those bulk emails will not create issues/comments.