XI2 relative motion events documentation

[Rendaw]

  I played around with the valuator data a bit.  In a XI_RawMotion 
event, the valuators.values and corresponding raw_values both return 
relative motion data, but they are way different.  I'm not sure by how 
much exactly, but for my mouse the raw_values were several orders of 
magnitude smaller.  However, the valuators.values were in turn smaller 
than the actual cursor coordinate changes, usually by about half, 
although sometimes (randomly, even at larger values) they were equal.

Also perhaps of note, only the slave device reported raw events when 
there was the WM's passive grab (when I had a button down) in place.  
Only the master device reported raw events otherwise.  I'm not sure what 
to make of that, although I guess it makes my life easier for now.

Anyways, the valuator data is better than nothing, but I'm not sure how 
to interpret it if it is neither the final motion data nor the raw 
motion data.