Clarified documentation on context listener.
Clarified documentation on context listener.

MetricsServletContextListener does not instantiate any registries of its own, it merely injects provided registries into the servlet context.
mveitas/metrics,AltitudeDigital/metrics,gburton1/metrics,wfxiang08/metrics,gburton1/metrics,ohr/metrics,unitsofmeasurement/metrics,mattnelson/metrics,randomstatistic/metrics,valery1707/dropwizard-metrics,mspiegel/metrics,slovdahl/metrics,infusionsoft/yammer-metrics,thelastpickle/metrics,mnuessler/metrics,fcrepo4-archive/metrics,ind9/metrics,kevintvh/metrics,dropwizard/metrics,jplock/metrics,mtakaki/metrics,signalfx/metrics,infusionsoft/yammer-metrics,cirrus-dev/metrics,timezra/metrics,tempredirect/metrics,ChetnaChaudhari/metrics,fcrepo4-archive/metrics,jasw/metrics,mt0803/metrics,bentatham/metrics,Banno/metrics,rexren/metrics,dropwizard/metrics,egymgmbh/metrics,mveitas/metrics,slachiewicz/metrics,chenxianghua2014/metrics,dropwizard/metrics