Fixed #18478 - Documented how to use a mutable default in a model field.

The default value for the field. This can be a value or a callable object. If

callable it will be called every time a new object is created.

The default cannot be a mutable object (model instance, list, set, etc.), as a

reference to the same instance of that object would be used as the default

value in all new model instances. Instead, wrap the desired default in a

callable.  For example, if you had a custom ``JSONField`` and wanted to specify

a dictionary as the default, use a ``lambda`` as follows::

    contact_info = JSONField("ContactInfo", default=lambda:{"email": "to1@example.com"})

``editable``

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