[RFC] Rewrite of DRS and SLP documentation files.

I spent some time rewriting the drs and slp format files.
DRS file is basically done, i think. But the SLP file needs some work - I'm not convinced by the SDL Command section, it seems overly verbose and repeats information

All comments appreciated
you corrected spec_color to special_colo(u)r below, but here it's still called spec colour.