criu(8): describe --weak-sysctls

This is an addition to commit 6273a5c ("Add ability to restore with
missing sysctls on the target node").

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>

Documentation/criu.txt

@@ -442,6 +442,11 @@ during *dump*, and images are migrated to a less capable CPU and are
 to be restored. By default, *criu* shows an error that CPU capabilities
 are not adequate, but this can be suppressed by using *--cpu-cap=none*.
 
+*--weak-sysctls*::
+   Silently skip restoring sysctls that are not available. This allows
+   to restore on an older kernel, or a kernel configured without some
+   options.
+
 *check*
 ~~~~~~~
 Checks whether the kernel supports the features needed by *criu* to