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