doc: update keepalive.md to add why (#3993)
doc: update keepalive.md to add why (#3993)


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