Slightly rewrite @permalink and get_absolute_url() documentation.

Part 2 of the model instance documentation changes. Slightly tidied up
get_absolute_url() and @permalink documentation to collapse some of the
earlier versions into a preferred learning order. I'm still not
amazingly happy with this, but larger rewrites are needed to the URLconf
stuff across a few files before I can get it into the most natural
order, I suspect. That's a slightly longer-term project.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16702 bcc190cf-cafb-0310-a4f2-bffc1f526a37