Made a bunch of edits to docs/topics/cache.txt, mostly based on stuff from the...

Made a bunch of edits to docs/topics/cache.txt, mostly based on stuff from the Django Book. Backport of [10055] from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10305 bcc190cf-cafb-0310-a4f2-bffc1f526a37