Document how Mail Queue works, including how JIRA prioritizes notifications when more than one is triggered for the same user ( like in Edit issue)

When performing several changes in an issue through a screen (like the Edit screen), this might trigger several notifications at the same time for the same user, this notifications reach the Mail Queue and , at the time of flushing the mail queue, JIRA follows some unclear rules to create one consolidated notification containing all of them (it seems to appear as a new notification with an 'envelope' icon screenshot-1.png , but this is purely a conjecture and it might be just JIRA indicating that it is flushing the notifications one by one) using one of the notifications to be the 'title' (like 'Assigned' or 'Updated') but will also include all the other changes made in it.

When multiple notifications for the same user are in the Mail Queue, by testing it seems the current behavior to choose the 'title' is:

1. If an "Issue Assigned" is among the notifications, it is chosen to be the title.
2. If there is no "Issue Assigned" notification, an "Issue updated" notification will be sent with all the changes.
3. For some reason, if one of the changes made is Work logged added, it triggers a completely separate notification in adition to the "assigned" or "updated" notification, and might even contain changes that are alread included in them ( see example, the due date and assignee change ware included in both notifications screenshot-2.png)
4. Comments added through a screen do not trigger the "Commented notification" nor are included in the list of changes made.

The suggestion is to make a dedicated documentation page for the JIRA Mail Queue (as there is none to be found at the moment), including the rules and priorities for the notifications.

Notes:

• All the tests were done using the 'Reporter' as test subject. All changes to the issue were done by an admin user.
• The test was done in clean JIRA Software Server 7.3.8 and 7.4.0.
• Changing the order of the fields in the screen does not seem to affect the behavior.