clientv3: clarify Watch close conditions

The "too slow" comment is rather vague. If the server closes
the watch for being too slow (it doesn't seem to any more), the
watch client should gracefully resume instead of forcing the
user to handle it.

Also removed the 'opts' comment since it wasn't being maintained.

clientv3/watch.go

@@ -40,10 +40,9 @@ type WatchChan <-chan WatchResponse
 
 type Watcher interface {
 	// Watch watches on a key or prefix. The watched events will be returned
-	// through the returned channel.
-	// If the watch is slow or the required rev is compacted, the watch request
-	// might be canceled from the server-side and the chan will be closed.
-	// 'opts' can be: 'WithRev' and/or 'WithPrefix'.
+	// through the returned channel. If revisions waiting to be sent over the
+	// watch are compacted, then the watch will be canceled by the server, the
+	// client will post a compacted error watch response, and the channel will close.
 	Watch(ctx context.Context, key string, opts ...OpOption) WatchChan
 
 	// Close closes the watcher and cancels all watch requests.