Update integration documentation for gRPC and cl-protobufs
Update integration documentation for gRPC and cl-protobufs

cl-protobufs and gRPC are designed to work well together,
it is not required that they are both used in conjunction.
We should spell that out clearly, and describe in
both README's how the intergation works.

PiperOrigin-RevId: 399984076

qitab/grpc,qitab/grpc