clientv3: document context to Snapshot streaming API

Signed-off-by: Gyuho Lee <[email protected]>

clientv3/maintenance.go

@@ -55,6 +55,11 @@ type Maintenance interface {
 	HashKV(ctx context.Context, endpoint string, rev int64) (*HashKVResponse, error)
 
 	// Snapshot provides a reader for a point-in-time snapshot of etcd.
+	// If the context "ctx" is canceled or timed out, reading from returned
+	// "io.ReadCloser" would error, and its error type is "grpc/*status.statusError".
+	// The error message would be either:
+	//  1. "rpc error: code = Canceled desc = context canceled"
+	//  2. "rpc error: code = DeadlineExceeded desc = context deadline exceeded"
 	Snapshot(ctx context.Context) (io.ReadCloser, error)
 
 	// MoveLeader requests current leader to transfer its leadership to the transferee.