[1.7.x] Fixed #12008 -- Clarified relationship between template blocks and includes.

Thanks Daniele Procida for suggested wording.

Backport of 80b8d3bee0 from master
[1.7.x] Fixed #23615 -- Validate that a Model instance's "check" attribute is a method.

The "check" name is a reserved word used by Django's check framework,
and cannot be redefined as something else other than a method, or the check
framework will raise an error.

This change amends the django.core.checks.model_check.check_all_models()
function, so that it verifies that a model instance's attribute "check"
is actually a method. This new check is assigned the id "models.E020".

Conflicts:
	docs/ref/checks.txt

Backport of a5c77417a6 from master
[1.7.x] Fixed #23667 -- Incorrect settings reference for language cookie.

Thanks jamesbeith for the report.

Backport of bedd439297 from master
[1.7.x] Fixed some intersphinx references.

Backport of babbf18999 from master