[try 4] Add help message box to dxdiag implementation

[Dan Kegel]

Looks fine to me.  FWIW, I'd suggest continuing on with a
second patch that adds one more feature, and post the
two-patch series.  Also, you're welcome to send it to
Austin and me first for pre-review.

[I 'spose you should have mentioned it's identical to the previous try.]