Ticket 20870 - document @cached_property, restructure topics/cache.txt

There's a fuller explanation of ``@cached_property`` in ref/utils.txt, and a pointer to that from topics/cache.txt.

The latter has been restructured so that general optimisation notes are not buried at the bottom of the caching framework section. Future notes on optimisation will find a place in the ``General optimization tools`` section.

https://code.djangoproject.com/ticket/20870
https://groups.google.com/forum/#!topic/django-developers/MytkDIgZ9kA