Good idea.  I'll write up a file-level docstring describing WCS in general and the conventions adopted in the file about (u,v), etc.  

However, if you don't mind, I'll do that on the main #364 branch rather than here.  The doc string for BaseWCS itself has been significantly expanded between here (where it is almost nothing) and there (where it goes for 2 pages or so).  Likewise, I have worked on the various function doc strings a fair bit as well, so I may have already addressed some of your concerns about this.