Describe the -no-emul-boot mkisofs(8) option and use it in the example.

Also remove the -U option from the example, since it could create ISO
images that some systems could not boot from (besides, it is not used in
the FreeBSD src/release/i386/mkisoimages.sh script, and there probably
is a reason for that ;)

PR: 70952
Submitted by: Jon Wallace <jon@whoweb.com>
Based on: patch by Sebastian Schulze Struchtrup <seb@struchtrup.com>