Documentation: cfq-iosched: update documentation help for cfq tunables

Add the documentation text for latency, target_latency & group_idle
tunnable parameters in the block/cfq-iosched.txt.
Also fix few typo(spelling) mistakes.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Amit Sahrawat <a.sahrawat@samsung.com>

Language somewhat modified by Jens.

Signed-off-by: Jens Axboe <axboe@kernel.dk>