Clarify database selection when a queryset is used in an {{{__in}}} lookup:

[Lev Zlobin]

• Without an explicitly selected database, the subquery uses the outer query's connection rather than being routed independently.
• Explicitly selecting a different database for the inner queryset raises ValueError.
• Evaluating the inner queryset with list() allows values to be retrieved from another database.

Add examples to the multiple-databases guide and cross-references from the using() and in lookup documentation.

This documentation improvement was split out from https://code.djangoproject.com/ticket/37427 following review on github ​https://github.com/django/django/pull/22154.