Fixed #19700 - Added documentation for BoundField.field.

    case, each object in the loop is a simple string containing the error

    message.

``field.is_hidden``

``{{ field.is_hidden }}``

    This attribute is ``True`` if the form field is a hidden field and

    ``False`` otherwise. It's not particularly useful as a template

    variable, but could be useful in conditional tests such as::

           {# Do something special #}

        {% endif %}

``{{ field.field }}``

    The :class:`~django.forms.Field` instance from the form class that

    this :class:`~django.forms.BoundField` wraps. You can use it to access

    :class:`~django.forms.Field` attributes , e.g.

    ``{{ char_field.field.max_length }}``.

Looping over hidden and visible fields

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