core: add clarification to ClientCall javadoc
core: add clarification to ClientCall javadoc

* Improve ClientCall documentation. Clarify that some methods of ClientCall may be invoked from any thread. Adds @throws clauses to request().

grpc/grpc-java,carl-mastrangelo/grpc-java,elandau/grpc-java,ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,ejona86/grpc-java,carl-mastrangelo/grpc-java,dapengzhang0/grpc-java,zhangkun83/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,zhangkun83/grpc-java,dapengzhang0/grpc-java,elandau/grpc-java,carl-mastrangelo/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,elandau/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,grpc/grpc-java