internal: tweak ClientTransport.GracefulClose documentation (#2772)
internal: tweak ClientTransport.GracefulClose documentation (#2772)

* internal: tweak ClientTransport.GracefulClose documentation

* address menghanl@ review comment

dfawley/grpc-go,menghanl/grpc-go,dfawley/grpc-go,grpc/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go