help: Update "Unsubscribe from a stream".

- Updates Desktop/web instructions to follow current help center
  documentation patterns.
- Adds instructions to unsubscribe from a stream via the stream settings
  panel.

help/unsubscribe-from-a-stream.md

@@ -4,19 +4,44 @@ You can always unsubscribe from any stream in Zulip.
 
 {start_tabs}
 
+{tab|desktop-web}
+
 {!stream-actions.md!}
 
 1. Click **Unsubscribe**.
 
 {end_tabs}
 
-## From the streams page (alternate method)
+## Alternate methods to unsubscribe from a stream
+
+### Via manage streams
 
 {start_tabs}
 
+{tab|desktop-web}
+
 {relative|stream|subscribed}
 
-1. Click on the green checkmark the left of the stream name.
+1. Click the green checkmark to the left of a stream to unsubscribe from it.
+
+{end_tabs}
+
+### Via stream settings
+
+{start_tabs}
+
+{tab|desktop-web}
+
+{!stream-actions.md!}
+
+1. Click **Stream settings**.
+
+1. Click **Unsubscribe** near the top right corner of the stream settings panel.
+
+!!! keyboard_tip ""
+
+    You can also use <kbd>Shift</kbd> + <kbd>S</kbd> to unsubscribe from the
+    selected stream.
 
 {end_tabs}