Rewrite the lazy model reference section of related fields docs

[Simon Charette]

This is follow up of the lazy operation refactor (#24215) that was initially thought to have introduced a regression in Django 1.9 (#25858) which ended up being a feature (#26186) people relied on since at least Django 1.2.

During the discussion about the behavior of app relative model reference defined on abstract models it was suggested ​the documentation should be revised to detail how ''recursive'', ''relative'' and ''absolute'' string references work and go on to explain how they work in an abstract context.