stat: add comments describing the quirky behavior of clat prio samples

stat: add comments describing the quirky behavior of clat prio samples

Commit 56440e63ac17 ("fio: report percentiles for slat, clat, lat")
together with commit 38ec5c514104 ("stat: make priority summary statistics
consistent with percentiles") changed so that per prio stats track either
completion latency (clat) or total latency (lat), depending on the option
lat_percentiles.

It is not obvious why add_clat_sample() shouldn't add a high/low clat prio
sample when option lat_percentiles is set, especially considering that
option lat_percentiles is usually used for controlling if total latency
percentiles should be displayed or not.

Add comments to describe why add_clat_sample() has to care about option
lat_percentiles.

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/20211125132020.109955-3-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>