Modernise/fix/rewrite unlinted manpages

Modernise/fix/rewrite unlinted manpages

zpool-destroy.8: flatten, fix description
zfs-wait.8: flatten, fix description, use list for events
zpool-reguid.8: flatten, fix description
zpool-history.8: flatten, fix description
zpool-export.8: flatten, fix description, remove -f "unmount" reference
  AFAICT no such command exists even in Illumos (as of today, anyway),
  and we definitely don't call it
zpool-labelclear.8: flatten, fix description
zpool-features.5: modernise
spl-module-parameters.5: modernise
zfs-mount-generator.8: rewrite
zfs-module-parameters.5: modernise

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12169