[skip ci] Add comment for addPChannels (#9173)

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

internal/querynode/collection.go

@@ -108,6 +108,7 @@ func (c *Collection) getVChannels() []Channel {
 	return c.vChannels
 }
 
+// addPChannels add physical channels to physical channels of collection
 func (c *Collection) addPChannels(channels []Channel) {
 OUTER:
 	for _, dstChan := range channels {