Add a comment why we are sending emails

ee/app/services/projects/prometheus/alerts/notify_service.rb

@@ -31,6 +31,11 @@ module Projects
         end
 
         def send_email?
+          # Send email if the `incident_management` feature flag is disabled.
+          # This is done in order to keep the old behavior of sending emails for
+          # any project which does not have the new `incident_management` feature.
+          # See point 3 in
+          # https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9830#what-does-this-mr-do
           return firings.any? unless incident_management_available?
 
           incident_management_setting.send_email && firings.any?