criu --help, Doc/criu.txt: fix -v option description

Current description of -v[vvv] was taken from criu-log.h comments
and describes specific log levels used by pr_* functions. The problem
is -vX includes all previous X-1, X-2... levels. Say, -vvvv description
says "debug only", while in fact it is not "only". Fix accordingly.

Also, removed -v0 description as it is useless. What -v0 in fact does is
it sets the log level to default -- same as if -vXXX is not used.

In addition, change a delimiter between option alternatives from comma
to a vertical bar in criu --help output, to be in line with the rest
of usage output.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>