Add a note about when to validate ping payloads (#57795)

Specifically, it will probably be important for #53657.

Author: amcasey

src/Servers/Kestrel/Core/src/Internal/Http2/Http2Connection.cs

@@ -1086,6 +1086,8 @@ private Task ProcessPingFrameAsync(in ReadOnlySequence<byte> payload)
  if (_incomingFrame.PingAck)
  {
  // TODO: verify that payload is equal to the outgoing PING frame
+ // This will be particularly important if we implement dynamic window sizing
+ // (https://github.com/dotnet/aspnetcore/issues/53657).
  return Task.CompletedTask;
  }