clientv3: document grpc-go error handling

[gyuho]

https://github.com/coreos/etcd/tree/master/clientv3#error-handling explains current error handling.
But we've upgraded gRPC to >v1.6. And lots of changes have happened in balancer/retrial logic.

With more fixes/changes to clientv3 on the way, now it's possible that clientv3 can receive errors from gRPC that are not grpc/status.(*statusError) type (e.g. grpc.ErrClientConnClosing in TestKVNewAfterClose, transport.ErrConnClosing in TestLeasingOwnerDeleteError).

c.f.

• are all errors returned by gRPC wrapped by status.Status? grpc/grpc-go#1581 (comment)
• Clarify/define clientv3 retry logic (fix broken retries) #8691

Either we convert the errors, or document clearly together with client upgrade guides.

[gyuho]

Partially covered by #8896.
Need update clientv3/doc.go on the gRPC connection errors.