minor: Add comment explaining list_render.get call.

It's a bit confusing when you read this code to know
where the original list was created.  I'm not a huge
fan of the cache scheme here, but it does seem to
work for live updates.

static/js/stream_ui_updates.js

@@ -163,6 +163,14 @@ exports.update_subscribers_list = function (sub) {
         $(".subscriber_list_settings_container").hide();
     } else {
         const emails = stream_edit.get_email_of_subscribers(sub.subscribers);
+
+        /*
+            We try to find a subscribers list that is already in the
+            cache that list_render.js maintains.  The list we are
+            looking for would have been created in the function
+            stream_edit.show_subscription_settings, using the same
+            naming scheme as below for the `name` parameter.
+        */
         const subscribers_list = list_render.get("stream_subscribers/" + sub.stream_id);
 
         // Changing the data clears the rendered list and the list needs to be re-rendered.