help: Document @ topic mentions.

Documents new wildcard mention for topic participants updating
and making tweaks to all relevant pages.

Fixes #27657.

Co-authored-by: Alya Abbott <alya@zulip.com>

help/dm-mention-alert-notifications.md

@@ -30,20 +30,22 @@ messages) from desktop notifications.
 
 ## Wildcard mentions
 
-By default, wildcard mentions (`@**all**`, `@**everyone**`, or `@**stream**`)
-trigger email/push notifications as though they were personal @-mentions.  You
-can toggle whether you receive notifications for wildcard mentions.
+By default, wildcard mentions (`@**all**`, `@**everyone**`, `@**stream**`,
+or `@**topic**`) trigger email/push notifications as though they were
+personal @-mentions. You can toggle whether you receive notifications
+for wildcard mentions.
 
 !!! tip ""
 
     Unlike personal mentions, wildcard mentions do not trigger notifications
-    in muted streams.
+    in muted streams or topics.
 
 {start_tabs}
 
 {settings_tab|notifications}
 
-1.  In the **Notification triggers** table, toggle the **@all** checkbox for **Streams**.
+1.  In the **Notification triggers** table, toggle the **@all** checkbox for
+    **Streams** or **Followed topics**.
 
 {end_tabs}
 

help/include/mentions-examples.md

@@ -4,7 +4,7 @@
 Users: @**Bo Lin** or @**Ariella Drake|26** (two `*`)
 User group: @*support team* (one `*`)
 Silent mention: @_**Bo Lin** or @_**Ariella Drake|26** (`@_` instead of `@`)
-Wildcard mentions: @**all**, @**everyone**, or @**stream** (two `*`)
+Wildcard mentions: @**all**, @**everyone**, @**stream**, or @**topic** (two `*`)
 ```
 
 !!! tip ""

help/include/mentions-intro.md

@@ -3,4 +3,6 @@ attention to a message. Mentions follow the same [notification
 settings](/help/dm-mention-alert-notifications) as direct messages and alert
 words. A [silent mention](/help/mention-a-user-or-group#silently-mention-a-user)
 allows you to refer to a user without triggering a notification. A wildcard
-mention allows you to [mention everyone on a stream](/help/mention-a-user-or-group#mention-everyone-on-a-stream).
+mention allows you to
+[mention everyone on a stream](/help/mention-a-user-or-group#mention-everyone-on-a-stream),
+or [mention all topic participants](/help/mention-a-user-or-group#mention-all-topic-participants).

help/mention-a-user-or-group.md

@@ -55,18 +55,37 @@ notification. Silent mentions start with `@_` instead of `@`.
 
 {end_tabs}
 
+## Mention all topic participants
+
+Using the `@**topic**` mention, you can mention everyone who has previously
+participated in the topic by sending a message or
+[reacting](/help/emoji-reactions) with an emoji. This lets you notify just those
+who have already engaged in the conversation.
+
+Users can disable notifications for `@**topic**` mentions by:
+
+- Disabling notifications for wildcard mentions
+[globally](/help/dm-mention-alert-notifications) or for [individual
+streams](/help/stream-notifications).
+
+- Muting the [topic](/help/mute-a-topic) or [stream](/help/mute-a-stream).
+
 ## Mention everyone on a stream
 
 You can mention everyone on a stream with the `@**all**` mention (or,
-equivalently, `@**everyone**` or `@**stream**`). Use sparingly! Used improperly,
-wildcard mentions can be annoying.
+equivalently, `@**everyone**` or `@**stream**`). It's best to use these mentions
+only when your message requires timely attention from many stream subscribers.
+An organization can be configured to
+[restrict](/help/restrict-wildcard-mentions) wildcard mentions in large streams.
 
-Note that this will not notify anyone who has muted the stream, and
-users can disable receiving email/push notifications for these
-wildcard mentions, either
+Users can disable notifications for `@**topic**` mentions by:
+
+- Disabling notifications for wildcard mentions
 [globally](/help/dm-mention-alert-notifications) or for [individual
 streams](/help/stream-notifications).
 
+- Muting the [topic](/help/mute-a-topic) or [stream](/help/mute-a-stream).
+
 ## Examples
 
 {!mentions-examples.md!}