Merge pull request #796 from azmeuk/issue-793-filters-doc

docs: improve filters documentation

src/wtforms/fields/core.py

@@ -56,7 +56,11 @@ def __init__(
         :param validators:
             A sequence of validators to call when `validate` is called.
         :param filters:
-            A sequence of filters which are run on input data by `process`.
+            A sequence of callable which are run by :meth:`~Field.process`
+            to filter or transform the input data. For example
+            ``StringForm(filters=[str.strip, str.upper])``.
+            Note that filters are applied after processing the default and
+            incoming data, but before validation.
         :param description:
             A description for the field, typically used for help text.
         :param id: