refs #20233 - Full custom user model example isn't really full

Addition and fix for custom user model example documentation.

https://code.djangoproject.com/ticket/20233

I added the missing fragment to the docs.

I also tested the docs and found another problem with this example. 
When you use the provided code, you will get this error:

    ImproperlyConfigured at /admin/
    'MyUserAdmin.fieldsets[3][1]['fields']' refers to field 'last_login' that is missing from the form.

It complains that we're referring to a missing field `last_login`.
In order to correct this, you can simply remove the offending line:

    ('Important dates', {'fields': ('last_login',)}),

I added both changes to the docs file.