criu --help: shorten --weak-symbols descr

Option --help should provide a brief information, like a list of
commands and options and their meaning, ideally no more than 1 line per
option. There is no need to describe nuances and details here, since
we have documentation (wiki and  man pages) for that.

travis-ci: success for Document --weak-syscalls
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/crtools.c

@@ -819,9 +819,7 @@ usage:
 "     --exec-cmd         execute the command specified after '--' on successful\n"
 "                        restore making it the parent of the restored process\n"
 "  --freeze-cgroup       use cgroup freezer to collect processes\n"
-"  --weak-sysctls        silently skip restoring sysctl if it is not available,\n"
-"                        these helps to overcome problems with restore on older\n"
-"                        kernel or with some kernel configuration disabled\n"
+"  --weak-sysctls        skip restoring sysctls that are not available\n"
 "\n"
 "* External resources support:\n"
 "  --external RES        dump objects from this list as external resources:\n"