config-linux: Add (array, optional) for linux.devices

To match the omitempty which the Go property has had since 28cc423
(add omitempty to 'Device' and 'Namespace', 2016-03-10, opencontainers#340).

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

config-linux.md

@@ -98,7 +98,7 @@ There is a limit of 5 mappings which is the Linux kernel hard limit.
 
 ## Devices
 
-`devices` is an array specifying the list of devices that MUST be available in the container.
+**`devices`** (array, optional) lists devices that MUST be available in the container.
 The runtime may supply them however it likes (with [mknod][mknod.2], by bind mounting from the runtime mount namespace, etc.).
 
 The following parameters can be specified: