diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 83ee0c8df..891b250f4 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -982,6 +982,9 @@ class ModelSerializer(Serializer):
 
         instance.save()
 
+        # Note that many-to-many fields are set after updating instance.
+        # Setting m2m fields triggers signals which could potentialy change
+        # updated instance and we do not want it to collide with .update()
         for attr, value in m2m_fields:
             field = getattr(instance, attr)
             field.set(value)