Driver: clarify help string for "-###"

Someone recently wasted some time not realising that "-###" didn't
actually execute the commands it printed, and suggested a
documentation tweak.

Having made the same mistake myself on at least one occasion, I
sympathise. So here it is. Any kibitzing on an even better text
welcome.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199256 91177308-0d34-0410-b5e6-96231b3b80d8