docs: turn off appcache on safari

Turns out AppCache on Safari 7 is totally broken (see
http://stackoverflow.com/questions/22888945/safari-7-application-cache-does-not-work).
This, combined with our "reload if you can't load CSS or JS" strategy for
multi-server deploy causes infinite reloads in some cases.

Hopefully this will resolve the sporadic complaints we get
from some users about infinite reloads.