doc: update the manpage document for file lock c/r

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Documentation/crtools.txt

@@ -106,6 +106,11 @@ OPTIONS
     Also this option allows to migrate a single external tty connection, in other
     words this option allows to migrate such application as 'top' and friends.
 
+-l|--file-locks::
+	Allow to dump and restore file locks. It's necessary to make sure that
+	all file lock users are taken into dump. So it's only safe to use this
+	for a container dump/restore.
+
 -V|--version::
     Print program version.