Fixed #10361 -- Added documentation for ComboField and MultiValueField form...

Takes the following optional arguments:

.. attribute:: URLField.max_length

.. attribute:: URLField.min_length

Slightly complex built-in ``Field`` classes

-------------------------------------------

The following are not yet documented.

``ComboField``

~~~~~~~~~~~~~~

.. class:: ComboField(**kwargs)

    * Default widget: ``TextInput``

    * Empty value: ``''`` (an empty string)

    * Normalizes to: A Unicode object.

    * Validates that the given value against each of the fields specified

      as an argument to the ``ComboField``.

    * Error message keys: ``required``, ``invalid``

Takes one extra required argument:

.. attribute:: ComboField.fields

    The list of fields that should be used to validate the field's value (in

    the order in which they are provided).

        >>> f = ComboField(fields=[CharField(max_length=20), EmailField()])

        >>> f.clean('test@example.com')

        u'test@example.com'

        >>> f.clean('longemailaddress@example.com')

        Traceback (most recent call last):

        ...

        ValidationError: [u'Ensure this value has at most 20 characters (it has 28).']

``MultiValuefield``

~~~~~~~~~~~~~~~~~~~

.. class:: MultiValueField(**kwargs)

    * Default widget: ``TextInput``

    * Empty value: ``''`` (an empty string)

    * Normalizes to: the type returned by the ``compress`` method of the subclass.

    * Validates that the given value against each of the fields specified

      as an argument to the ``MultiValueField``.

    * Error message keys: ``required``, ``invalid``

    This abstract field (must be subclassed) aggregates the logic of multiple

    fields. Subclasses should not have to implement clean(). Instead, they must

    implement compress(), which takes a list of valid values and returns a

    "compressed" version of those values -- a single value.  For example,

    :class:`SplitDateTimeField` is a subclass which combines a time field and

    a date field into a datetime object.

Takes one extra required argument:

.. attribute:: MultiValueField.fields

    A list of fields which are cleaned into a single field. Each value in

    ``clean`` is cleaned by the corresponding field in ``fields`` -- the first

    value is cleaned by the first field, the second value is cleaned by

    the second field, etc.  Once all fields are cleaned, the list of clean

    values is "compressed" into a single value.

``SplitDateTimeField``

~~~~~~~~~~~~~~~~~~~~~~

.. class:: SplitDateTimeField(**kwargs)

    * Default widget: ``SplitDateTimeWidget``

.. attribute:: ModelChoiceField.empty_label

    By default the ``<select>`` widget used by ``ModelChoiceField`` will have a

   an empty choice at the top of the list. You can change the text of this label

   (which is ``"---------"`` by default) with the ``empty_label`` attribute, or

   you can disable the empty label entirely by setting ``empty_label`` to

   ``None``::

    an empty choice at the top of the list. You can change the text of this

    label (which is ``"---------"`` by default) with the ``empty_label``

    attribute, or you can disable the empty label entirely by setting

    ``empty_label`` to ``None``::

        # A custom empty label

        field1 = forms.ModelChoiceField(queryset=..., empty_label="(Nothing)")