Improve the default parallel GC settings, and sanitise the flags (#3340)

Improve the default parallel GC settings, and sanitise the flags (#3340)

Flags (from +RTS -?):

  -qg[<n>]  Use parallel GC only for generations >= <n>
            (default: 0, -qg alone turns off parallel GC)
  -qb[<n>]  Use load-balancing in the parallel GC only for generations >= <n>
            (default: 1, -qb alone turns off load-balancing)

these are good defaults for most parallel programs.  Single-threaded
programs that want to make use of parallel GC will probably want +RTS
-qg1 (this is documented).

I've also updated the docs.