gRPC examples and clarifications

[dmuelle]

During the development of the gRPC topic in #2698 @hutchig identified a few enhancements that might improve the topic but that did not constitute a stop-ship to publish it. Recording them here so we can add to the topic in a future release.

• What is not clear in the docs is if the (server) grpc feature enables 100% of the grpcClient feature's features. (i.e. a server can always also be a client without having grpcClient in the server.xml.)

• Regarding 'it becomes accessible to remote gRPC clients on the configured HTTP ports.' I think it might help to have an example to clarify, if there is any, gRPC bits of the port config and if so how to do this.

• Does having "headersToPropagate="cookie"." have an effect to 'propagate security tokens?

• provide some further snippets of relevant actual config elements that fit in with the text.