criu --help: improve --inherit-fd description

Add that --inherit-fd may also access file_path argument, such as
in this example from wiki (see
https://criu.org/Inheriting_FDs_on_restore#Regular_files):

	$ ./test.sh > /tmp/old &
	<pid>
	$ sudo criu dump -j -t <pid>
	$ sudo criu restore -d -j --inherit-fd 'fd[7]:tmp/old' 7> /tmp/new

travis-ci: success for Add/fix description of --external and --inherit-fd
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/crtools.c

@@ -875,6 +875,7 @@ usage:
 "                            pipe[inode]\n"
 "                            socket[inode]\n"
 "                            file[mnt_id:inode]\n"
+"                            path/to/file\n"
 "  --empty-ns net        Create a namespace, but don't restore its properties\n"
 "                        (assuming it will be restored by action scripts)\n"
 "  -J|--join-ns NS:{PID|NS_FILE}[,OPTIONS]\n"