Fixed #24019 -- Fixed inaccurate docs about GenericRelation not supporting aggregation.

This works at least as far back as Django 1.6 according to the test
added in refs #10870.

docs/ref/contrib/contenttypes.txt

@@ -450,18 +450,12 @@ signal.
 Generic relations and aggregation
 ---------------------------------
 
-:doc:`Django's database aggregation API </topics/db/aggregation>`
-doesn't work with a
+:doc:`Django's database aggregation API </topics/db/aggregation>` works with a
 :class:`~django.contrib.contenttypes.fields.GenericRelation`. For example, you
-might be tempted to try something like::
+can find out how many tags all the bookmarks have::
 
-    Bookmark.objects.aggregate(Count('tags'))
-
-This will not work correctly, however. The generic relation adds extra filters
-to the queryset to ensure the correct content type, but the
-:meth:`~django.db.models.query.QuerySet.aggregate` method doesn't take them
-into account. For now, if you need aggregates on generic relations, you'll
-need to calculate them without using the aggregation API.
+    >>> Bookmark.objects.aggregate(Count('tags'))
+    {'tags__count': 3}
 
 .. module:: django.contrib.contenttypes.forms