Update add-unit helptext
Update add-unit helptext

Refs https://bugs.launchpad.net/juju-core/+bug/1567925

macgreagoir/juju,bz2/juju,bz2/juju,AdamIsrael/juju,bac/juju,perrito666/juju,dooferlad/juju,ericsnowcurrently/juju,marcmolla/juju,bac/juju,mikemccracken/juju,AdamIsrael/juju,howbazaar/juju,anastasiamac/juju,bogdanteleaga/juju,reedobrien/juju,dimitern/juju,axw/juju,frankban/juju,gabriel-samfira/juju,waigani/juju,bac/juju,mikemccracken/juju,waigani/juju,bogdanteleaga/juju,kat-co/juju,gabriel-samfira/juju,dooferlad/juju,bz2/juju,dooferlad/juju,gabriel-samfira/juju,axw/juju,dimitern/juju,mjs/juju,marcmolla/juju,howbazaar/juju,mjs/juju,bz2/juju,anastasiamac/juju,mjs/juju,marcmolla/juju,axw/juju,ericsnowcurrently/juju,reedobrien/juju,ericsnowcurrently/juju,mjs/juju,AdamIsrael/juju,dimitern/juju,kat-co/juju,marcmolla/juju,alesstimec/juju,frankban/juju,makyo/juju,alesstimec/juju,ericsnowcurrently/juju,bz2/juju,frankban/juju,mikemccracken/juju,bac/juju,reedobrien/juju,anastasiamac/juju,alesstimec/juju,anastasiamac/juju,gabriel-samfira/juju,kat-co/juju,howbazaar/juju,AdamIsrael/juju,dooferlad/juju,makyo/juju,alesstimec/juju,dooferlad/juju,kat-co/juju,reedobrien/juju,waigani/juju,perrito666/juju,mikemccracken/juju,makyo/juju,anastasiamac/juju,axw/juju,makyo/juju,dimitern/juju,frankban/juju,macgreagoir/juju,bogdanteleaga/juju,mjs/juju,macgreagoir/juju,ericsnowcurrently/juju,gabriel-samfira/juju,bac/juju,mjs/juju,mikemccracken/juju,perrito666/juju,waigani/juju,perrito666/juju,kat-co/juju,mjs/juju,macgreagoir/juju,dimitern/juju,howbazaar/juju,axw/juju,frankban/juju,howbazaar/juju,macgreagoir/juju,alesstimec/juju,reedobrien/juju,marcmolla/juju,bogdanteleaga/juju,perrito666/juju