api: Clarify Server APIs can be used before start()
api: Clarify Server APIs can be used before start()

Fixes #8349

dapengzhang0/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,grpc/grpc-java,ejona86/grpc-java,grpc/grpc-java