help: Document "Who can unsubscribe others from this stream?" setting.

- Updates "Remove others" row in "Stream permissions" tables.
- Adds a section to "Add or remove users from a stream" describing
  how to configure the new setting.
- Documents the new setting in "Create a stream".
- Documents "Message retention period" setting in "Create a stream".

Fixes #24340.

help/add-or-remove-users-from-a-stream.md

@@ -90,6 +90,29 @@ This method is useful if you need to remove one user from multiple streams.
 
 {end_tabs}
 
+## Configure who can remove users
+
+{!admin-only.md!}
+
+Organization administrators can configure who can remove other users from a
+public stream. For private streams, administrators must be subscribed to the
+stream to configure this setting.
+
+{start_tabs}
+
+{relative|stream|all}
+
+1. Select a stream.
+
+{!select-stream-view-general.md!}
+
+1. Under **Stream permissions**, configure
+   **Who can unsubscribe others from this stream?**
+
+{!save-changes.md!}
+
+{end_tabs}
+
 ## Related articles
 
 * [Browse and subscribe to streams](/help/browse-and-subscribe-to-streams)