help: Document how notifications work for edited and deleted messages.

help/delete-a-message.md

@@ -83,6 +83,14 @@ time, they are permanently and irrecoverably deleted from the Zulip
 server.  Server administrators can adjust the archival time using
 the `ARCHIVED_DATA_VACUUMING_DELAY_DAYS` setting.
 
+## Message notifications
+
+If you delete a message soon after sending it, any [pending email
+notifications](/help/email-notifications#delay-before-sending-emails)
+for that message will be canceled, and
+[visual desktop notifications](/help/desktop-notifications) will be removed,
+including [mentions and alerts](/help/dm-mention-alert-notifications).
+
 ## Related articles
 
 * [Delete a topic](/help/delete-a-topic)

help/edit-a-message.md

@@ -48,6 +48,23 @@ edit the content of a message sent by another user.
     [view a message's edit history](/help/view-a-messages-edit-history)
     if it is [enabled](/help/disable-message-edit-history) in your organization.
 
+## Message notifications
+
+If you edit a message to [mention a user or group](/help/mention-a-user-or-group),
+the newly mentioned users will receive notifications just as if they had been
+mentioned in the original message.
+
+If you edit a message soon after sending it, the edit will be reflected in any
+[email notifications that have not yet been sent](/help/email-notifications#delay-before-sending-emails).
+This includes canceling notifications for users whose
+[mention](/help/format-your-message-using-markdown#mention-a-user-or-group) was
+removed or changed from a regular mention to a
+[silent mention](/help/mention-a-user-or-group#silently-mention-a-user).
+
+If you [delete the content of a message](/help/delete-a-message#delete-message-content),
+any pending email notifications for that message will be canceled, including
+[mentions and alerts](/help/dm-mention-alert-notifications).
+
 ## Related articles
 
 * [View the Markdown source of a message](/help/view-the-markdown-source-of-a-message)