Reflect f2330bd156
change in our man pages and add some context.
Wording is primarily copy-pasted from code comments.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes#16581