don't repeat benchmark docs in 3 places
don't repeat benchmark docs in 3 places

make Usage() the single place where arguments are described.

raskle/pilosa,travisturner/pilosa,raskle/pilosa,alanbernstein/pilosa,linhvo/pilosa,linhvo/pilosa,alanbernstein/pilosa,alanbernstein/pilosa,raskle/pilosa,linhvo/pilosa