Move some details of CheckMessage to the reference guide.

[Jack Laxson]

​https://docs.djangoproject.com/en/1.8/topics/checks/ doesn't say where django looks for these checks... it looks like checks.py is the right answer.

edit: So It appears that you need to hook the checks into django yourself? (e.g. via Appconfig) This seems like a high bar of entry but it needs to be documented better

[Tim Graham]

I think it's documented pretty well in ​Registering and labeling checks, but feel free to suggest a patch if something is unclear.

[Baptiste Mispelon]

The "Registering and labeling checks" section has the right information indeed but it's not that obvious that you should read it.

I think the problem is the reference documentation of CheckMessage that sits between the code example that uses @register without telling you where the code should live, and the paragraph that suggests you to put the code in an AppConfig.ready() method.
Personally, it made me switch between the "read as text, from top to bottom" mode (ie "topic-style" documentation) to the "hop around trying to find the info I'm looking for" mode (ie "reference-style" documentation).

Could we maybe move the bulk of the reference documentation for CheckMessage into the /ref/ documentation section?

[Tim Graham]

​Proposal

[Tim Graham <timograham@…>]

In faa2a0f:

Fixed #25174 -- Moved some details of CheckMessage to the reference guide.

[Tim Graham <timograham@…>]

In 386a6dc3:

[1.8.x] Fixed #25174 -- Moved some details of CheckMessage to the reference guide.

Backport of faa2a0f662ed6fe0b90d10e98cc8ee3795d9307c from master