Be very clear about when it's appropriate to use defer() and only().

I've been seeing a bit of over-reliance on defer() and only() in code
around the place and it's generally better modelled with normalised data
or shadow (unmanaged) models. This commit makes this position clearer.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16692 bcc190cf-cafb-0310-a4f2-bffc1f526a37