Revise docs regarding non-int primary key on custom user models

[shadfc]

The docs (​https://docs.djangoproject.com/en/1.8/topics/auth/customizing/#specifying-a-custom-user-model, point #1) say that a custom user model must have an integer primary key but do not specify why. https://code.djangoproject.com/ticket/24161 dealt with a session issue and has been fixed.

I would like to be able to use a UUID as the pk for a custom user model but this message makes me unsure if I should attempt that. Tim Graham suggested opening a ticket to investigate this issue and either remove that note from the docs or add a better explanation to it.

[Claude Paroz]

The fact that we have tests for an UUIDUser (see #24161) should prove that the integer PK is no longer a requirement.

I'd suggest:

docs/topics/auth/customizing.txt

@@ -469 +469 @@
 
 Django expects your custom User model to meet some minimum requirements.
 
-1. Your model must have an integer primary key.
-
-2. Your model must have a single unique field that can be used for
+1. Your model must have a single unique field that can be used for
    identification purposes. This can be a username, an email address,
    or any other unique attribute.
 
-3. Your model must provide a way to address the user in a "short" and
+2. Your model must provide a way to address the user in a "short" and
    "long" form. The most common interpretation of this would be to use
    the user's given name as the "short" identifier, and the user's full
    name as the "long" identifier. However, there are no constraints on

[Tim Graham]

I thought we might want to do a bit more thorough test/investigating instead of relying only on reports of things that *didn't* work.

[Tim Graham <timograham@…>]

In 981e3b9:

Fixed #24429 -- Doc'ed that Django 1.8 doesn't require an integer PK for custom user models.

[Tim Graham <timograham@…>]

In 993074d:

[1.8.x] Fixed #24429 -- Doc'ed that Django 1.8 doesn't require an integer PK for custom user models.

Backport of 981e3b9394022810bd536474344e2f91c190b858 from master

[Tim Graham]

For posterity, I didn't do any further investigation. Let's see if we get any reports about things that don't work.