block: add documentation for inflight

Complements v2.6.32 commit a9327cac ("Seperate read and write
statistics of in_flight requests") and commit 316d315b ("block:
Seperate read and write statistics of in_flight requests v2").

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20211019130230.77594-1-maier@linux.ibm.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>