Provide an articulate example of how to properly delete partitions and

partitioning scheme.

Users often get confused and frustrated when trying to delete partition
table and getting ``Device busy'' error because they forgot (or did not
ever know that they have) to delete all its partitions first, and while
the manual page mentions this briefly, it does not stress it out enough.

Approved by:		ae, manpages (bjk)
PR (as inspiration):	196102
Differential Revision:	https://reviews.freebsd.org/D12336

sbin/geom/class/part/gpart.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 25, 2016
+.Dd September 19, 2017
 .Dt GPART 8
 .Os
 .Sh NAME
@@ -1325,6 +1325,29 @@ After creating all required partitions, embed bootstrap code into them:
 .Bd -literal -offset indent
 /sbin/gpart bootcode -p /boot/boot1 da0
 .Ed
+.Ss Deleting partitions and partitioning scheme
+If you get a
+.Em "Device busy"
+error when trying to destroy a partition table, remember that you must
+delete all its partitions first with the
+.Cm delete
+action.
+In this example, assume we have
+.Pa da0
+with three partitions:
+.Bd -literal -offset indent
+/sbin/gpart delete -i 3 da0
+/sbin/gpart delete -i 2 da0
+/sbin/gpart delete -i 1 da0
+/sbin/gpart destroy da0
+.Ed
+.Pp
+Alternatively, you can invoke the
+.Cm destroy
+action with the
+.Fl F
+flag.
+.Ed
 .Ss Backup and Restore
 .Pp
 Create a backup of the partition table from