help: Add mobile instructions to "Unsubscribe from a stream".

- Documents unsubscribing via long-press menu, all streams, and
  stream settings menu.

help/unsubscribe-from-a-stream.md

@@ -10,6 +10,14 @@ You can always unsubscribe from any stream in Zulip.
 
 1. Click **Unsubscribe**.
 
+{tab|mobile}
+
+{!stream-long-press-menu.md!}
+
+1. Tap **Unsubscribe**.
+
+{!stream-long-press-menu-tip.md!}
+
 {end_tabs}
 
 ## Alternate methods to unsubscribe from a stream
@@ -24,6 +32,19 @@ You can always unsubscribe from any stream in Zulip.
 
 1. Click the green checkmark to the left of a stream to unsubscribe from it.
 
+{tab|mobile}
+
+{!mobile-all-streams-view.md!}
+
+1. Tap the **checkmark**
+   (<img src="/static/images/help/mobile-check-icon.svg" alt="checkmark" class="mobile-icon"/>)
+   icon to the right of a stream to unsubscribe from it.
+
+!!! tip ""
+
+    You can also press and hold a stream until the long-press menu appears, and
+    tap **Unsubscribe**.
+
 {end_tabs}
 
 ### Via stream settings
@@ -43,6 +64,16 @@ You can always unsubscribe from any stream in Zulip.
     You can also use <kbd>Shift</kbd> + <kbd>S</kbd> to unsubscribe from the
     selected stream.
 
+{tab|mobile}
+
+{!stream-long-press-menu.md!}
+
+1. Tap **Stream settings**.
+
+1. Tap **Unsubscribe**.
+
+{!mobile-stream-settings-menu-tip.md!}
+
 {end_tabs}
 
 ## Related articles