[1.6.x] Fixed #23597 -- Clarified the manager that {Single,Multiple}ObjectMixin.model uses.

Backport of e501d4c505 from master
[1.6.x] Fixed #23604 -- Allowed related m2m fields to be references in the admin.

Thanks Simon Charette for review.

Backport of a24cf21722 from master
[1.6.x] Updated links in admin docs to use :ref:

per Sphinx docs: "Using ref is advised over standard reStructuredText
links to sections because it works across files, when section headings
are changed, and for all builders that support cross-references.

Backport of 13409b932a from master