Document the ability to omit the router-id.
Document the ability to omit the router-id.

woniullb/babeld,sudomesh/babeld,wlanslovenija/babeld,dtaht/babeld-shortrtt-metrics,tcatm/babeld,Gwendocg/babeldToS,Drooids/babeld,jech/babeld,wlanslovenija/babeld,boutier/babeld,jech/babeld,Gwendocg/babeldToS