Fix SelectFieldBase.iter_choices docstring

This fixes one of the out-of-date docstrings mentioned in pallets-eco#811 but not fixed in pallets-eco#812.

src/wtforms/fields/choices.py

@@ -30,7 +30,7 @@ def __init__(self, label=None, validators=None, option_widget=None, **kwargs):
     def iter_choices(self):
         """
         Provides data for choice widget rendering. Must return a sequence or
-        iterable of (value, label, selected) tuples.
+        iterable of (value, label, selected, render_kw) tuples.
         """
         raise NotImplementedError()