Document NULL handling of exclude() with __in subqueries

[verajs]

The in field lookup reference (docs/ref/models/querysets.txt) documents
passing a queryset on the right-hand side of __in, but does not mention
that exclude(field__in=qs) where qs.values(...) can return NULL produces
a silently-empty result set on Postgres (and any database following SQL's
three-valued logic).

This is the subquery counterpart of #20024 (literal-list flavor, fixed in
cec10f99). The subquery flavor was identified by Anssi Kääriäinen and Simon
Riggs on django-developers in 2013 [1], but the ORM-side fix never landed,
and there is currently no warning in the docs.

We hit this in production on a real invoice queryset and tracked it down to
the documented NOT IN / NULL semantics. The workaround we shipped is the
same one Simon Riggs recommended in the 2013 thread: rewrite to
~Exists(OuterRef(...)), which compiles to NULL-safe NOT EXISTS.

Proposal: add a .. warning:: block to the in field lookup reference that

1. describes the failure mode (NULL in subquery -> NOT IN evaluates to UNKNOWN -> empty queryset)
2. points readers to Exists + OuterRef as the NULL-safe replacement
3. cross-references #20024 for the literal-list case

Plus a one-line cross-reference under the exclude() method docs pointing
to the new warning.

[1] ​https://groups.google.com/g/django-developers/c/OG1unUV-MOU

[verajs]

I have a patch ready:
​https://github.com/django/django/pull/21414