Add documentation for new population metrics
Add documentation for new population metrics

Signed-off-by: Deepak Giridharagopal <d11186354d1ef01ca06ae37d7e23e827da13e85f@puppetlabs.com>

ajroetker/puppetdb,johnduarte/puppetdb,jantman/puppetdb,shrug/puppetdb,kbrezina/puppetdb,kbarber/puppetdb,ajroetker/puppetdb,grimradical/puppetdb,jantman/puppetdb,mullr/puppetdb,github/puppetlabs-puppetdb,johnduarte/puppetdb,melissa/puppetdb,grimradical/puppetdb,grimradical/puppetdb,cprice404/puppetdb,github/puppetlabs-puppetdb,rbrw/puppetdb,highb/puppetdb,shrug/puppetdb,johnduarte/puppetdb,kbrezina/puppetdb,kbrezina/puppetdb,haus/puppetdb,cprice404/puppetdb,wkalt/puppetdb,haus/puppetdb,rbrw/puppetdb,waynr/puppetdb,cprice404/puppetdb,waynr/puppetdb,waynr/puppetdb,rbrw/puppetdb,nfagerlund/puppetdb,haus/puppetdb,highb/puppetdb,wkalt/puppetdb,ajroetker/puppetdb,melissa/puppetdb,ajroetker/puppetdb,highb/puppetdb,jantman/puppetdb,highb/puppetdb,senior/puppetdb,shrug/puppetdb,mullr/puppetdb,melissa/puppetdb,kbarber/puppetdb,kbarber/puppetdb,nfagerlund/puppetdb,rbrw/puppetdb,grimradical/puppetdb,senior/puppetdb,shrug/puppetdb,mullr/puppetdb,johnduarte/puppetdb,mullr/puppetdb,puppetlabs/puppetdb,kbarber/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,kbrezina/puppetdb,rbrw/puppetdb,mullr/puppetdb,puppetlabs/puppetdb,stahnma/puppetdb,wkalt/puppetdb,stahnma/puppetdb,senior/puppetdb,nfagerlund/puppetdb,waynr/puppetdb,wkalt/puppetdb,github/puppetlabs-puppetdb,stahnma/puppetdb,puppetlabs/puppetdb,senior/puppetdb