Corrected links to only()/defer() in Model documentation

Specifying ``update_fields`` will force an update.

When saving a model fetched through deferred model loading

(:meth:`~Model.only()` or :meth:`~Model.defer()`) only the fields loaded from

the DB will get updated. In effect there is an automatic ``update_fields`` in

this case. If you assign or change any deferred field value, these fields will

be added to the updated fields.

(:meth:`~django.db.models.query.QuerySet.only()` or

:meth:`~django.db.models.query.QuerySet.defer()`) only the fields loaded

from the DB will get updated. In effect there is an automatic

``update_fields`` in this case. If you assign or change any deferred field

value, the field will be added to the updated fields.

Deleting objects

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