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>

Documentation/block/bfq-iosched.txt

@@ -357,6 +357,13 @@ video playing/streaming, a very low drop rate may be more important
 than maximum throughput. In these cases, consider setting the
 strict_guarantees parameter.
 
+slice_idle_us
+-------------
+
+Controls the same tuning parameter as slice_idle, but in microseconds.
+Either tunable can be used to set idling behavior.  Afterwards, the
+other tunable will reflect the newly set value in sysfs.
+
 strict_guarantees
 -----------------