Document how to pass a list of arguments to a single parameter key

[wiwa]

For example, with --scalac-options, the only documentation I could find looked like --scalac-options -Ywarn-unused. If I had 2 options, A and B, I wouldn't know how to pass them as a list of options. I would (somewhat non-intuitively) have to say --scalac-options A --scalac-options B. We should consider documenting this.