[django-filter] Improve stubtest comment (#14738)

Author: srittau

stubs/django-filter/@tests/stubtest_allowlist.txt

@@ -15,6 +15,5 @@ django_filters.fields.RangeField.__init__
 django_filters.filters.QuerySetRequestMixin.__init__
 django_filters.widgets.CSVWidget.__init__
 
-# FIXME: new stubtest errors from mypy v1.18.1 that need to be looked at more closely.
-# See https://github.com/python/typeshed/pull/14699
+# BaseCSVFilter constructs a custom class dynamically, which stubtest can't handle.
 django_filters(\.\w+)*\.OrderingFilter.field_class

stubs/django-filter/django_filters/filters.pyi

@@ -292,7 +292,10 @@ class LookupChoiceFilter(Filter):
     def filter(self, qs: QuerySet[Any], lookup: Lookup) -> QuerySet[Any]: ...
 
 class OrderingFilter(BaseCSVFilter, ChoiceFilter):
-    field_class: type[BaseCSVField]  # Inherits CSV field behavior for comma-separated ordering
+    # Inherits CSV field behavior for comma-separated ordering.
+    # BaseCSVFilter constructs a custom ConcreteCSVField class that derives
+    # from BaseCSVField.
+    field_class: type[BaseCSVField]
     descending_fmt: str
     param_map: dict[str, str] | None
     def __init__(