Clarify the effects of changing primary keys in models and ModelForms

[Sarah Boyce]

The primary key documentation explains that changing an existing instance’s primary key and saving it creates a new object alongside the original. See ​https://docs.djangoproject.com/en/dev/topics/db/models/#field-options

However, it does not explain that if an object with the new primary key already exists, saving the instance updates that object instead.

This behavior also applies to a ModelForm bound to an existing instance when an editable primary key field is included in the form. Changing that field can cause the form to update a different object from the one originally supplied through instance.

We should clarify this existing behavior by:

• Extending the primary key example to demonstrate updating an object that already has the newly assigned primary key. A model with an additional, non-primary-key field would help illustrate which data is updated.
• Expanding ​Selecting the fields to use to explain the consequences of including an editable primary key field, with a link to the model documentation. Applications should exclude this field unless users are intended to be able to change it.