Document the fact that we only need an NSRunLoop in the example because there isn't one already.

examples/hello_world.rs

@@ -61,6 +61,8 @@ fn main() -> Result<(), Error> {
     }
     tts.speak("Goodbye.", false)?;
     let mut _input = String::new();
+    // The below is only needed to make the example run on MacOS because there is no NSRunLoop in this context.
+    // It shouldn't be needed in an app or game that almost certainly has one already.
     #[cfg(target_os = "macos")]
     {
         let run_loop: id = unsafe { NSRunLoop::currentRunLoop() };