criu(8): use list for -v arguments

Also, s/options/arguments/.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Documentation/criu.txt

@@ -93,8 +93,10 @@ OPTIONS
     Write logging messages to 'file'.
 
 *-v* 'num'::
-    Set logging level to 'num'. Valid options are: *0* - (silent, error messages
-    only), *1* - informative (default), *2* - debug messages.
+    Set logging level to 'num'. Valid arguments are:
+        * *0* - (silent, error messages only);
+        * *1* - informative (default);
+        * *2* - debug messages.
 
 *--log-pid*::
     Write separate logging files per each pid.