Fixed #8705: Corrected some formatting issues in models documentation

ones:

    :attr:`~Field.null`

        If ``True``, Django will store empty values as ``NULL`` in the database.

        Default is ``False``.

    :attr:`~Field.blank`

        If ``True``, the field is allowed to be blank. Default is ``False``.

        Note that this is different than :attr:`~Field.null`.

        <Field.blank>`, the field will be required.

    :attr:`~Field.choices`

        An iterable (e.g., a list or tuple) of 2-tuples to use as choices for

        this field. If this is given, Django's admin will use a select box

        instead of the standard text field and will limit choices to the choices

            )

    :attr:`~Field.default`

        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.

    :attr:`~Field.help_text`

        Extra "help" text to be displayed under the field on the object's admin

        form. It's useful for documentation even if your object doesn't have an

        admin form.

    :attr:`~Field.primary_key`

        If ``True``, this field is the primary key for the model.

        If you don't specify :attr:`primary_key=True <Field.primary_key>` for

        see :ref:`automatic-primary-key-fields`.

    :attr:`~Field.unique`

        If ``True``, this field must be unique throughout the table.

Again, these are just short descriptions of the most common field options. Full