SignalR docs: verify we point out MessagePack response is binary

[wadepickett]

Verify the SignalR docs either point back to details on MessagePack that indicate responses are binary, or point it out directly where needed.

We do point it out here: Use MessagePack Hub Protocol in SignalR for ASP.NET Core - What is MessagePack?

See dotnet/aspnetcore#43455 Contributes to #27021