[vtkusers] vtkRenderWindow contents to ppm

[Michael Halle]

Since I had the same sort of question about a month ago about
vtkRenderWindow and saving images, I did take the time to update the
documentation for that class to include information about the change.

I would suggest that this point be added to an expanded, on-line
viewable "converting from vtk3.2 to vtk4.0" document (the currently
available (useful!) information is in a zip file you have to
download).

							--Mike