block: doc: add slice_idle_us to bfq documentation

Of the tunables available for the bfq I/O scheduler, the only one
missing from the documentation in 'Documentation/block/bfq-iosched.txt'
is slice_idle_us. Add this tunable to the documentation and a short
explanation of its purpose.

Acked-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>