Clarify the Javadoc for CallStreamObserver

The relevant part of the current Javadoc:
https://github.com/grpc/grpc-java/blob/6b04fc18f30deb3519c389fe457b99cf5a8d99d0/stub/src/main/java/io/grpc/stub/CallStreamObserver.java#L25-L35

As far as I can tell, for any call, there are three implementations of `StreamObserver` involved:
1) Implemented by the application, on the RPC client side ('inbound')
2) Implemented by the framework, on the RPC client side ('outbound'), `ClientCallStreamObserver`
3) Implemented by the framework, on the RPC server side ('outbound'), `ServerCallStreamObserver`

I think it would be nice to clarify that. I've spent significant time trying to understand where does `ClientCallStreamObserver` appear, being confused by the premise that there are only two implementations of `StreamObserver`.

Start with stub/src/main/java/io/grpc/stub/CallStreamObserver.java, especially the Javadoc at lines 25–35 linked in the issue. Clarify the three StreamObserver implementations and the roles of ClientCallStreamObserver and ServerCallStreamObserver; the Javadoc should make where ClientCallStreamObserver appears understandable.

索引モデルが issue の本文から書いたものです。