refs: document how ref_iterator_advance_fn should handle symrefs

Signed-off-by: Han-Wen Nienhuys <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: hanwen

refs/refs-internal.h

@@ -438,6 +438,11 @@ void base_ref_iterator_free(struct ref_iterator *iter);
 
 /* Virtual function declarations for ref_iterators: */
 
+/*
+ * backend-specific implementation of ref_iterator_advance.
+ * For symrefs, the function should set REF_ISSYMREF, and it should also dereference
+ * the symref to provide the OID referent.
+ */
 typedef int ref_iterator_advance_fn(struct ref_iterator *ref_iterator);
 
 typedef int ref_iterator_peel_fn(struct ref_iterator *ref_iterator,