doc/dev/cache-pool: fix notes

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1329 from ceph/wip-doc-cache-pool

fix up docs for cache pools

These are definitely not complete, but they bring a few of the commands more up to date with current syntax.

Reviewed-by: Greg Farnum <greg@inktank.com>