Document everything available in service handler context

[dfawley]

As requested in #2626, we should document all the fields available via the context as they are not really discoverable. The doc should include grpc functions like SetHeader/Method/etc as well as fields stored directly within the context like metadata and peer info.

[codemaveric]

Hi Doug,

I will like to help document all the various fields available via the request Context. I will need a little bit of guidance to document it properly.

[aranjans]

Hi I'd be interested in resolving this issue.

[dfawley]

Sure, that would be helpful, thanks!