Fixed #22064 -- Add check for related_name

Fixed #23602 -- Add comment on get_absolute_url regarding user input

Thanks Troy Grosfield for the report and Tim Graham for the tests.

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.

Thanks Simon Charette for review.

Updated assertQuerysetEqual documentation following 311b3ad9

Validates that related_name is a valid Python id or ends with a '+' and
it's not a keyword. Without a check it passed silently leading to
unpredictable problems.

Thanks Konrad Świat for the initial work.

This reverts commit 6fa9fa91.

Aymeric: "I chose not to talk about django.setup() here on purpose.
This will hardly always be the correct solution."

4ad57bbe included a few odd
quotation marks.

Thanks to Tim Graham for the review.

Enabling the attribute causes get_object() to perform its lookup
using both the primary key and the slug.

Thanks tchaumeny and Loic for reviews.

Thanks Claude Paroz, Riccardo Magliocchetti, and Damian Moore
for contributions to the patch.

Forward port of e9c8aefb from stable/1.7.x

Thanks oinopion for the suggestion and jarus for the initial patch.

Thanks Collin Anderson for the review.