[skip ci] Add comment for collection id (#8806)

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

internal/querynode/collection.go

@@ -51,6 +51,7 @@ type Collection struct {
 	releaseTime        Timestamp
 }
 
+// ID returns collection id
 func (c *Collection) ID() UniqueID {
 	return c.id
 }