docs: document quirky implementation of per priority stats reporting

docs: document quirky implementation of per priority stats reporting

Commit 56440e63ac17 ("fio: report percentiles for slat, clat, lat") changed
many things. One of the changes, from the commit message:
"- for the new cmdprio_percentage latencies, if lat_percentiles=1,
*total* latency percentiles will be tracked. Otherwise, *completion*
latency percentiles will be tracked."

In other words, the commit changed the per prio stats from always tracking
(and reporting) clat latency, to instead either track (and report) clat or
lat latency.

Considering that a certain latency type reports two things:
1) min/max/avg latency for the the specific latency type
2) latency percentiles for the specific latency type

If disable_clat/disable_lat is used, neither 1) nor 2) will be reported.
If clat_percentiles/lat_percentiles is false, 2) will not be reported.

Therefore it is unintuitive that setting lat_percentiles=1, an option
usually used to enable/disable percentile reporting, also affects which
type of latency that will be tracked (and reported) for per prio stats.

The fact that the variables are named e.g. clat_prio_stat, regardless of
the type of latency being tracked does not help.

Anyway, let's document the way that the current implementation works,
so that a user can know how per priority stats are handled, without having
to read the source, since the commit that introduced this behavior forgot
to update the documentation.

Fixes: 56440e63ac17 ("fio: report percentiles for slat, clat, lat")
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20211125132020.109955-2-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>