help: Document Markdown formatting for wildcard mentions.

- Documents wildcard mentions (`@all`, `@everyone`, `@stream`),
  linking to /help/mention-a-user-or-group#mention-everyone-on-a-stream
  from the intro text.

Fixes #25783.

help/dm-mention-alert-notifications.md

@@ -30,8 +30,8 @@ messages) from desktop notifications.
 
 ## Wildcard mentions
 
-By default, wildcard mentions (`@**all**`, `@**everyone**`) trigger
-email/push notifications as though they were personal @-mentions.  You
+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.
 
 !!! tip ""

help/include/mentions-examples.md

@@ -4,6 +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 `*`)
 ```
 
 !!! tip ""

help/include/mentions-intro.md

@@ -2,4 +2,5 @@ You can mention a team member or [user group](/help/user-groups) to call their
 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.
+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).