bcache: documentation updates and corrections

Bcache documentation updates:
- Added new HOWTO/COOKBOOK section
- fixed a few typos
- /sys/block/bcache0/cache_mode is /sys/block/bcache0/bcache/cache_mode

Signed-off-by: Marc MERLIN <marc@merlins.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>