Fix #21594 -- Add note about model formsets deleting objects

Some users may interpret `formset.save(commit=False)` to mean that no changes are committed to the database, when deletions actually are. (This behavior is fixed in ticket [#10284](https://code.djangoproject.com/ticket/10284) / commit 65e03a424e82e157.)

See Track ticket [#21594](https://code.djangoproject.com/ticket/21594).