Document changes in default Email Alarm Callback

Refs #2868

UPGRADING.rst

@@ -11,4 +11,12 @@ for the complete upgrade notes.
 From 2.1 to 2.2
 ===============
 
-tba
+Email Alarm Callback
+--------------------
+
+In previous Graylog versions, creating an alert condition on a stream and adding an alert receiver meant that, if no alarm callback existed for that stream, Graylog would use an Email alarm callback by default.
+
+Due to the extensive rework done in alerting, this behaviour has been modified to be explicit, and more consistent with other entities within Graylog: from now on **there will not be a default alarm callback**.
+
+To easy the transition to people relying on this behaviour, we have added a migration step that will create an Email alarm callback for each stream that has alert conditions, has alert receivers, but has no associated alarm callbacks.
+