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

man/man8/zpool-export.8

@@ -18,7 +18,6 @@
 .\"
 .\" CDDL HEADER END
 .\"
-.\"
 .\" Copyright (c) 2007, Sun Microsystems, Inc. All Rights Reserved.
 .\" Copyright (c) 2012, 2018 by Delphix. All rights reserved.
 .\" Copyright (c) 2012 Cyril Plisko. All Rights Reserved.
@@ -30,24 +29,17 @@
 .Dd February 16, 2020
 .Dt ZPOOL-EXPORT 8
 .Os
+.
 .Sh NAME
 .Nm zpool-export
-.Nd Exports the given ZFS storage pools from the system
+.Nd export ZFS storage pools
 .Sh SYNOPSIS
 .Nm zpool
 .Cm export
-.Op Fl a
 .Op Fl f
-.Ar pool Ns ...
+.Fl a Ns | Ns Ar pool Ns …
+.
 .Sh DESCRIPTION
-.Bl -tag -width Ds
-.It Xo
-.Nm zpool
-.Cm export
-.Op Fl a
-.Op Fl f
-.Ar pool Ns ...
-.Xc
 Exports the given pools from the system.
 All devices are marked as exported, but are still considered in use by other
 subsystems.
@@ -69,15 +61,12 @@ the disks.
 .It Fl a
 Exports all pools imported on the system.
 .It Fl f
-Forcefully unmount all datasets, using the
-.Nm unmount Fl f
-command.
-This option is not supported on Linux.
+Forcefully unmount all datasets, and allow export of pools with active shared spares.
 .Pp
 This command will forcefully export the pool even if it has a shared spare that
 is currently being used.
 This may lead to potential data corruption.
 .El
-.El
+.
 .Sh SEE ALSO
 .Xr zpool-import 8