help-docs: Update wildcards for new channel wildcard.

Updates the mention everyone wildcards for the new channel wildcard
and removes references to the stream wildcard.

help/dm-mention-alert-notifications.md

@@ -30,7 +30,7 @@ messages) from desktop notifications.
 
 ## Wildcard mentions
 
-By default, wildcard mentions (`@**all**`, `@**everyone**`, `@**stream**`,
+By default, wildcard mentions (`@**all**`, `@**everyone**`, `@**channel**`,
 or `@**topic**`) trigger email/push notifications as though they were
 personal @-mentions. You can toggle whether you receive notifications
 for wildcard mentions.

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**, @**stream**, or @**topic** (two `*`)
+Wildcard mentions: @**all**, @**everyone**, @**channel**, @**topic** (two `*`)
 ```
 
 !!! tip ""

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

@@ -73,13 +73,13 @@ streams](/help/stream-notifications).
 ## Mention everyone on a stream
 
 You can mention everyone on a stream with the `@**all**` mention (or,
-equivalently, `@**everyone**` or `@**stream**`). It's best to use these mentions
+equivalently, `@**everyone**` or `@**channel**`). 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.
 
 Users can disable notifications for `@**all**`, `@**everyone**`, and
-`@**stream**` mentions by:
+`@**channel**` mentions by:
 
 - Disabling notifications for wildcard mentions
 [globally](/help/dm-mention-alert-notifications) or for [individual