Fixed #21120 -- Added more explicit text on using validators and link to writing validators.

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Django's form (and model) fields support use of simple utility functions and

classes known as validators. These can be passed to a field's constructor, via

the field's ``validators`` argument, or defined on the Field class itself with

the ``default_validators`` attribute.

classes known as validators. A validator is merely a callable object or

function that takes a value and simply returns nothing if the value is valid or

raises a :exc:`~django.core.exceptions.ValidationError` if not. These can be

passed to a field's constructor, via the field's ``validators`` argument, or

defined on the :class:`~django.forms.Field` class itself with the

``default_validators`` attribute.

Simple validators can be used to validate values inside the field, let's have

a look at Django's ``SlugField``::

    slug = forms.CharField(validators=[validators.validate_slug])

Common cases such as validating against an email or a regular expression can be

handled using existing validator classes available in Django. For example,

``validators.validate_slug`` is an instance of

a :class:`~django.core.validators.RegexValidator` constructed with the first

argument being the pattern: ``^[-a-zA-Z0-9_]+$``. See the section on

:doc:`writing validators </ref/validators>` to see a list of what is already

available and for an example of how to write a validator.

Form field default cleaning

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