add reference to msc3768

Signed-off-by: Kerry Archibald <kerrya@element.io>

proposals/3767-time-based-notification-filtering.md

@@ -71,11 +71,18 @@ For example, Wednesday morning focus time rule:
             },
     ],
     "actions": [
-        "dont_notify"
+        "dont_notify" // See note below
     ]
 }
 ```
 
+**`dont_notify` and Do Not Disturb behaviour**
+`dont_notify` will stop badges from being
+updated in the client during 'do not disturb' hours, so the user will not be
+able to locate messages that were silenced when they are back online.
+`notify_in_app` as proposed in
+[MSC3768](https://github.com/matrix-org/matrix-spec-proposals/pull/3768) should
+be used.
 
 ## Potential issues
 - If a user changes timezone their push rules will not automatically update.