document complex code related with _created management

Author: bagerard

mongoengine/base/document.py

@@ -202,6 +202,8 @@ def __setattr__(self, name, value):
             and self__created
             and name == self._meta.get("id_field")
         ):
+            # When setting the ID field of an instance already instantiated and that was user-created (i.e not saved in db yet)
+            # Typically this is when calling .save()
             super().__setattr__("_created", False)
 
         super().__setattr__(name, value)