Document filters

[azmeuk]

We should better document filters. For the moment this does not really explain what they do and how to use them:

wtforms/src/wtforms/fields/core.py

Lines 58 to 59 in 0524993

	:param filters:
	A sequence of filters which are run on input data by `process`.

Though, inline filters have slightly better documentation:

wtforms/docs/forms.rst

Lines 149 to 158 in 0524993

	The same principle applies for filters with the convention `filter_fieldname`::
	class SignupForm(Form):
	name = StringField('name')
	def filter_name(form, field):
	return field.strip()
	Note that filters are applied after processing the default and incoming data,
	but before validation.