Docs: Streaming timeout with HttpClient

[JamesNK]

See protobuf-net/protobuf-net.Grpc#210

When a channel is customized to use a HttpClient and has log running streaming calls then HttpClient.Timeout will be triggered. Add section to troubleshooting doc.

[Drawaes]

I recently hit this issue and found this and the protobuf-net info after a lot of searching, reverting to just pushing in a handler instead of a whole http client was the fix for me also so a documentation note would be great.

[veselfil]

We have also ran into this recently. If you can point me to where in the docs this should go, I'd be happy to contribute the change.