Fixed #23754 -- Always allow reference to the primary key in the admin

This change will allow dynamically created inlines "Add related" button to
work correcly as long as their associated foreign key is pointing to the primary
key of the related model.

Also adjusted `get_inline_instances()` documentation to deal with another
possible edge case.

Thanks to amorce for the report, Julien Phalip for the initial patch
and Collin Anderson for the review.
Sigh...