[dm-cache] update documentation
[dm-cache] make constructor comment match cache.txt (formatting)
Merge branch 'thin-dev' of git://github.com/snitm/linux into cache-review-work2
[dm-cache] add some DM_ prefixes to #defines
[dm-cache] I see no need to pass the set_config through to the policy.
[dm-cache] tidy dm_cache_init
[dm-cache] fluff