document/fix use of annotations in distinct()

[jieter]

requires stable and unique names for annotations.

[jieter]

For example:
Doing <<queryset with join>>.order_by('title_i18n').distict('title_i18n') doesn't work, because distinct arguments are not rewritten.

Possible workaround is

<<queryset with join>>
    .annotate(title_order=models.functions.Cast('title_i18n', output_field=models.CharField(255)))
    .order_by('title_order')
    .distinct('title_order')

but it's better to avoid it.

[jieter]

Test case:
English works, but dutch fails with: ProgrammingError: SELECT DISTINCT ON expressions must match initial ORDER BY expressions.

        with override('en'):
            qs = Blog.objects.filter(category__name_i18n='Birds').order_by('title_i18n').distinct('title')
            self.assertEqual(key(qs, 'title_i18n', sep=' '), 'Falcon Vulture')

        with override('nl'):
            qs = Blog.objects.filter(category__name_i18n='Vogels').order_by('title_i18n').distinct('title_i18n')
            self.assertEqual(key(qs, 'title_i18n', sep=' '), 'Valk Gier')