Document what a "subcommand" is
Document what a "subcommand" is

There are a few references to "subcommands" in the repository, but no
explanation of what they are. Beef up the docblock for
`shouldRunAsSubcommand()` to make things a little clearer.
Specifically, I'd like to make the difference between driver jobs and
driver subcomnmands clearer.

In addition, remove a double negative comment, "If we are not run as
'swift', don't do anything special", and replace it with something a
little clearer (in my opinion).

jmgc/swift,nathawes/swift,gregomni/swift,harlanhaskins/swift,gregomni/swift,parkera/swift,CodaFi/swift,atrick/swift,ahoppen/swift,harlanhaskins/swift,CodaFi/swift,hooman/swift,parkera/swift,jmgc/swift,tkremenek/swift,roambotics/swift,stephentyrone/swift,stephentyrone/swift,atrick/swift,rudkx/swift,parkera/swift,allevato/swift,atrick/swift,tkremenek/swift,xwu/swift,hooman/swift,gregomni/swift,benlangmuir/swift,ahoppen/swift,hooman/swift,tkremenek/swift,atrick/swift,xwu/swift,ahoppen/swift,rudkx/swift,apple/swift,glessard/swift,xwu/swift,jckarter/swift,tkremenek/swift,glessard/swift,JGiola/swift,jckarter/swift,jmgc/swift,nathawes/swift,nathawes/swift,CodaFi/swift,roambotics/swift,rudkx/swift,rudkx/swift,parkera/swift,jmgc/swift,airspeedswift/swift,aschwaighofer/swift,aschwaighofer/swift,benlangmuir/swift,rudkx/swift,xwu/swift,aschwaighofer/swift,glessard/swift,airspeedswift/swift,airspeedswift/swift,apple/swift,atrick/swift,benlangmuir/swift,JGiola/swift,parkera/swift,gregomni/swift,ahoppen/swift,nathawes/swift,xwu/swift,jckarter/swift,xwu/swift,glessard/swift,roambotics/swift,JGiola/swift,parkera/swift,hooman/swift,airspeedswift/swift,xwu/swift,hooman/swift,airspeedswift/swift,harlanhaskins/swift,nathawes/swift,ahoppen/swift,benlangmuir/swift,aschwaighofer/swift,tkremenek/swift,atrick/swift,airspeedswift/swift,ahoppen/swift,jmgc/swift,rudkx/swift,harlanhaskins/swift,jmgc/swift,allevato/swift,apple/swift,nathawes/swift,aschwaighofer/swift,gregomni/swift,stephentyrone/swift,jmgc/swift,glessard/swift,CodaFi/swift,apple/swift,apple/swift,tkremenek/swift,allevato/swift,glessard/swift,harlanhaskins/swift,hooman/swift,JGiola/swift,airspeedswift/swift,tkremenek/swift,allevato/swift,CodaFi/swift,allevato/swift,stephentyrone/swift,parkera/swift,parkera/swift,jckarter/swift,roambotics/swift,stephentyrone/swift,JGiola/swift,gregomni/swift,stephentyrone/swift,CodaFi/swift,nathawes/swift,stephentyrone/swift,harlanhaskins/swift,jckarter/swift,aschwaighofer/swift,CodaFi/swift,apple/swift,allevato/swift,jckarter/swift,jckarter/swift,hooman/swift,benlangmuir/swift,JGiola/swift,aschwaighofer/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,harlanhaskins/swift,allevato/swift