doc/configuration: document optional index in NetworkPowerPort

Signed-off-by: Trevor Gamblin <[email protected]>

Author: threexc

doc/configuration.rst

@@ -146,7 +146,7 @@ The example describes port 0 on the remote power switch
 Arguments:
   - model (str): model of the power switch
   - host (str): hostname of the power switch
-  - index (int): number of the port to switch
+  - index (int, default=0): optional, number of the port to switch
 
 The ``model`` property selects one of several `backend implementations
 <https://github.com/labgrid-project/labgrid/tree/master/labgrid/driver/power>`_.