[SignalR] How Hubs interact with HttpContext

We should have a document describing the complex interations between Hubs and `HttpContext`.

* In a Hub you can get an `HttpContext` via `Context.GetHttpContext()` extension method, but it's not guaranteed to work
* Long Polling uses a "simulated" context
* There's no context at all when using Azure SignalR
* `IHttpContextAccessor` should be avoided
* SignalR has some support for non-HTTP transports (nothing officially supported yet but some future ideas) and those wouldn't even have an `HttpContext` so it's considered a good design practice to avoid using `HttpContext` at all in `Hub`s (to allow portability to non-HTTP transports)