Add @OptIn annotations to dev guide snippets

These cover developer journeys that we don't plan to stabilise in 1.0.0

PiperOrigin-RevId: 438812300

docs/listening-to-player-events.md

@@ -88,8 +88,6 @@ public void onPlayerError(PlaybackException error) {
   if (cause instanceof HttpDataSourceException) {
     // An HTTP error occurred.
     HttpDataSourceException httpError = (HttpDataSourceException) cause;
-    // This is the request for which the error occurred.
-    DataSpec requestDataSpec = httpError.dataSpec;
     // It's possible to find out more about the error both by casting and by
     // querying the cause.
     if (httpError instanceof HttpDataSource.InvalidResponseCodeException) {