Document deriv, update/correct docs for rate and delta.
Document deriv, update/correct docs for rate and delta.

ChaoticMind/docs,prometheus/docs,tcolgate/docs,evanphx/docs,dato/prometheus-docs,bluecmd/prometheus-docs,ddfreyne/prometheus-docs,tcolgate/docs,ChaoticMind/docs,vic3lord/docs,dln/docs,Cepave/openfalcon.io,open-falcon/open-falcon.github.io,open-falcon/open-falcon.github.io,chosenken/docs,chosenken/docs,du2016/prometheus-docs,vic3lord/docs,chosenken/docs,chosenken/docs,ddfreyne/prometheus-docs,jimmidyson/docs,du2016/prometheus-docs,pdbogen/docs,fabxc/docs,dato/prometheus-docs,prometheus/docs,jmptrader/docs,jimmidyson/docs,dato/prometheus-docs,pdbogen/docs,jmptrader/docs,jimmidyson/docs,Cepave/openfalcon.io,du2016/prometheus-docs,dln/docs,Cepave/openfalcon.io,ddfreyne/prometheus-docs,tcolgate/docs,evanphx/docs,jmptrader/docs,bluecmd/prometheus-docs,vic3lord/docs,dln/docs,pdbogen/docs,fabxc/docs,open-falcon/open-falcon.github.io,bluecmd/prometheus-docs,ChaoticMind/docs,evanphx/docs,prometheus/docs,ddfreyne/prometheus-docs,fabxc/docs