spec: Clarify use of metadata (headers) in clients

Hi!

An issue was raised [1] against `improbable-eng/grpc-web` asking what the best way is for a client to specify the HTTP/2 `:authority` header for routing between services in the backend. The spec mentions a few reserved headers, but I think it would be nice to have clarity on exactly which headers have a special meaning for the gRPC protocol and how they should be specified in a HTTP/1.1 world.

Should the client simply specify the metadata header as `authority: `?

1: https://github.com/improbable-eng/grpc-web/issues/266#issuecomment-432512985