Consistency and improvements to the form fields document.

[Antoliny]

Hello!
I created an issue by finded the wrong parts and the good parts when improved in the Form fields document.

1. ​Field.requried section invalid example.

>>> from django import forms
>>> f = forms.CharField()
...
>> f.clean(" ")
' '

In the example passed a character string with spaces as a factor in the clean method to a CharField instance.
This part is cleared space by CharField's to_python method, result is an empty_value so occur validation error.

class CharField(Field):
    ...
    def to_python(self, value):
        """Return a string."""
        if value not in self.empty_values:
            value = str(value)
            if self.strip:
                value = value.strip() # before value: "  ", after value: "" 
        if value in self.empty_values:
            return self.empty_value
        return value
    ...

However, in the current document, a string containing spaces seems to pass through the clean method.

---

2. ​Field.label section unclear part -> ("outputting forms as HTML" above)

It is unclear what " 'Outputting forms as HTML' above" refers to in the Field label section.
This part probably means the ​"Outputting forms as HTML" section of the Forms API Document.

---

3. Consistency "fallback" and link message

This part is very trivial :)

The word "fallback" used in the ​field initial section are two values "fallback" and "fall back".

"Also note that initial values are not used as "fallback" ...
---
>>> f.is_valid()
False
# The form does *not* fall back to using ....
...

I think it would be better to include documentation in the message set as a link.
(Like the link message used in ​validators section.)

``localize``
------------

.. attribute:: Field.localize

The ``localize`` argument enables the localization of form data input, as well
as the rendered output.

See the :doc:`format localization </topics/i18n/formatting>` documentation for # --> See the :doc:`format localization </topics/i18n/formatting> documentation` for more information.