[skip ci]Add comment for AddIndex in indexcoord (#12977)

Signed-off-by: Cai.Zhang <cai.zhang@zilliz.com>

internal/indexcoord/meta_table.go

@@ -141,6 +141,7 @@ func (mt *metaTable) reloadMeta(indexBuildID UniqueID) (*Meta, error) {
 	return m, nil
 }
 
+// AddIndex adds the index meta corresponding the indexBuildID to meta table.
 func (mt *metaTable) AddIndex(indexBuildID UniqueID, req *indexpb.BuildIndexRequest) error {
 	mt.lock.Lock()
 	defer mt.lock.Unlock()