Adjust comment for highlightsVisibleChanged

Remove the note about being able to toggle highlights from the host
frame, as the purpose of that note was unclear. Also add the context
that the event is only sent if the command to toggle highlights
originated from this guest.

src/types/port-rpc-events.d.ts

@@ -19,8 +19,9 @@ export type GuestToHostEvent =
   | 'anchorsChanged'
 
   /**
-   * Visibility of highlights was toggled from the guest frame (it can also be
-   * toggled from the host frame).
+   * Visibility of highlights was toggled from the guest frame. This event is
+   * not sent if highlights are turned on/off in the frame in response to a
+   * command from the sidebar or host frames.
    */
   | 'highlightsVisibleChanged';