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/zfs-wait.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.
@@ -27,25 +26,20 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd August 9, 2019
+.Dd May 31, 2021
 .Dt ZFS-WAIT 8
 .Os
+.
 .Sh NAME
 .Nm zfs-wait
-.Nd Wait for background activity to stop in a ZFS filesystem
+.Nd wait for activity in ZFS filesystem to stop
 .Sh SYNOPSIS
 .Nm zfs
 .Cm wait
-.Op Fl t Ar activity Ns Oo , Ns Ar activity Ns Oc Ns ...
-.Ar fs
+.Op Fl t Ar activity Ns Oo , Ns Ar activity Ns Oc Ns …
+.Ar filesystem
+.
 .Sh DESCRIPTION
-.Bl -tag -width Ds
-.It Xo
-.Nm zfs
-.Cm wait
-.Op Fl t Ar activity Ns Oo , Ns Ar activity Ns Oc Ns ...
-.Ar fs
-.Xc
 Waits until all background activity of the given types has ceased in the given
 filesystem.
 The activity could cease because it has completed or because the filesystem has
@@ -58,13 +52,14 @@ immediately.
 These are the possible values for
 .Ar activity ,
 along with what each one waits for:
-.Bd -literal
-        deleteq       The filesystem's internal delete queue to empty
-.Ed
+.Bl -tag -compact -offset Ds -width "deleteq"
+.It Sy deleteq
+The filesystem's internal delete queue to empty
+.El
 .Pp
 Note that the internal delete queue does not finish draining until
 all large files have had time to be fully destroyed and all open file
 handles to unlinked files are closed.
-.El
+.
 .Sh SEE ALSO
 .Xr lsof 8