Document broadcast banner in Git remote responses

doc/user/admin_area/broadcast_messages.md

@@ -20,10 +20,19 @@ You can style a message's content using the `a` and `br` HTML tags. The `br` tag
 
 ## Banners
 
-Banners are shown on the top of a page.
+Banners are shown on the top of a page and in Git remote responses.
 
 ![Broadcast Message Banner](img/broadcast_messages_banner_v12_10.png)
 
+```bash
+$ git push
+...
+remote:
+remote: **Welcome** to GitLab :wave:
+remote:
+...
+```
+
 ## Notifications
 
 Notifications are shown on the bottom right of a page and can contain placeholders. A placeholder is replaced with an attribute of the active user. Placeholders must be surrounded by curly braces, for example `{{name}}`.