It's been two releases since we started warning; time to make the
combination an error as promised.  There was no iotest coverage, so
add some.

While touching the documentation, tweak another section heading for
consistent style.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200706203954.341758-3-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>