[R] order of panels in xyplots

[Titus Malsburg]

The documentation of xyplot could be improved here.  It says:

  "If 'index.cond' is a list, it has to be as long as the number of conditioning
   variables, and the 'i'-th component has to be a valid indexing vector for the
   integer vector '1:nlevels(g_i)' (which can, among other things, repeat some
   of the levels or drop some altogether)."

It should make explicit that nlevels is the number of levels actually
used in the data and not length(levels(f)).

Cheers,

   Titus