Fixed #14820 -- Added more information to the generic relation docs regarding...

        2. Give your model a field that can store primary key values from the

           models you'll be relating to. For most models, this means a

           :class:`~django.db.models.PositiveIntegerField`.

           This field must be of the same type as the primary key of the models

           that will be involved in the generic relation. For example, if you use

           :class:`~django.db.models.fields.IntegerField`, you won't be able to

           form a generic relation with a model that uses a

           :class:`~django.db.models.fields.CharField` as a primary key.

           :class:`~django.db.models.PositiveIntegerField`. The usual name

           for this field is "object_id".

        3. Give your model a

           :class:`~django.contrib.contenttypes.generic.GenericForeignKey`, and

           :class:`~django.contrib.contenttypes.generic.GenericForeignKey` will

           look for.

.. admonition:: Primary key type compatibility

   The "object_id" field doesn't have to be the same type as the

   primary key fields on the related models, but their primary key values

   must be coercible to the same type as the "object_id" field by its

   :meth:`~django.db.models.Field.get_db_prep_value` method.

   For example, if you want to allow generic relations to models with either

   :class:`~django.db.models.IntegerField` or

   :class:`~django.db.models.CharField` primary key fields, you

   can use :class:`~django.db.models.CharField` for the

   "object_id" field on your model since integers can be coerced to

   strings by :meth:`~django.db.models.Field.get_db_prep_value`.

   For maximum flexibility you can use a

   :class:`~django.db.models.TextField` which doesn't have a

   maximum length defined, however this may incur significant performance

   penalties depending on your database backend.

   There is no one-size-fits-all solution for which field type is best. You

   should evaluate the models you expect to be pointing to and determine

   which solution will be most effective for your use case.

This will enable an API similar to the one used for a normal

:class:`~django.db.models.ForeignKey`;

each ``TaggedItem`` will have a ``content_object`` field that returns the