search: Improve text about searching public channels.

We use different text for guest users, specifying that they can
only search in channels they can view.
The issue description suggests the same for spectators, but
we already use different text for them.

The search-operators help overlay is also updated with the same.

Fixes #30902

web/src/ui_init.js

@@ -192,7 +192,11 @@ function initialize_compose_box() {
 }
 
 function initialize_message_feed_errors() {
-    $("#message_feed_errors_container").html(render_message_feed_errors());
+    $("#message_feed_errors_container").html(
+        render_message_feed_errors({
+            is_guest: current_user.is_guest,
+        }),
+    );
 }
 
 export function initialize_kitchen_sink_stuff() {