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

scripts/mancheck.sh

@@ -26,7 +26,7 @@ fi
 IFS="
 "
 
-files="$(find "$@" -type f -name '*[1-9]*' ! -name '*module-param*' ! -name 'zpool-features*' ! -name 'zfs-mount-generator*')" || exit 1
+files="$(find "$@" -type f -name '*[1-9]*')" || exit 1
 
 add_excl="$(awk '
     /^.\\" lint-ok:/ {