[KNIFE_RACKSPACE-1] improve server delete docs
[KNIFE_RACKSPACE-1] improve server delete docs

rackspace/knife-rackspace,stephenlauck/knife-rackspace,srirajan/knife-rackspace,vhl/knife-rackspace,chef/knife-rackspace,martinb3/knife-rackspace,decklin/knife-rackspace,tdooner/knife-rackspace,chef/knife-rackspace,philoserf/knife-rackspace