Fixed #25284 -- Documented removal of implicit QuerySet __in lookups.

Existing code that depends on ``list`` specific features, such as indexing,

can be ported by converting the iterator into a ``list`` using ``list()``.

Implicit ``QuerySet`` ``__in`` lookup removed

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In earlier versions, queries such as::

    Model.objects.filter(related_id=RelatedModel.objects.all())

would implicitly convert to::

    Model.objects.filter(related_id__in=RelatedModel.objects.all())

resulting in SQL like ``"related_id IN (SELECT id FROM ...)"``.

This implicit ``__in`` no longer happens so the "IN" SQL is now "=", and if the

subquery returns multiple results, at least some databases will throw an error.

Miscellaneous

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