help: Document stream notifications mobile feature.

- Documents how to set default notifications for all streams and
  single streams from within the mobile app.

help/stream-notifications.md

@@ -9,6 +9,8 @@ These settings will override any default stream notification settings.
 
 {start_tabs}
 
+{tab|desktop-web}
+
 {!stream-actions.md!}
 
 1. Click **Stream settings**.
@@ -18,6 +20,16 @@ These settings will override any default stream notification settings.
 1. Under **Notification settings**, toggle your preferred
    notifications settings for the stream.
 
+{tab|mobile}
+
+{!stream-long-press-menu.md!}
+
+1. Tap **Stream settings**.
+
+1. Toggle **Notifications**.
+
+{!mobile-stream-settings-menu-tip.md!}
+
 {end_tabs}
 
 ## Set default notifications for all streams
@@ -27,11 +39,24 @@ explicitly set a notification preference.
 
 {start_tabs}
 
+{tab|desktop-web}
+
 {settings_tab|notifications}
 
 1. In the **Notification triggers** table,
    toggle the settings for **Streams**.
 
+{tab|mobile}
+
+{!mobile-profile-menu.md!}
+
+1. Tap **Settings**.
+
+1. Tap **Notifications**.
+
+1. Toggle **Stream notifications** to configure mobile push notifications for
+   stream messages.
+
 {end_tabs}
 
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