vz.conf: add SKIP_ARPDETECT example

etc/vz.conf: add (commented out) SKIP_ARPDETECT with a short explanation
etc/vz.conf: improve ERROR_ON_ARPFAIL description in vz.conf
man/vz.conf(5): add SKIP_ARPDETECT, improve ERROR_ON_ARPFAIL description

Signed-off-by: Kir Kolyshkin <kir@openvz.org>