[skip ci] Add comment for getVChannels (#9172)

Signed-off-by: bigsheeper <yihao.dai@zilliz.com>

internal/querynode/collection.go

@@ -103,6 +103,7 @@ OUTER:
 	}
 }
 
+// getVChannels get virtual channels of collection
 func (c *Collection) getVChannels() []Channel {
 	return c.vChannels
 }