suzerain: #3036 better comment on form

I'd failed to comment on why exactly we can drop \Delta
into the governing equations given that the homogenized
terms have a leading y on them.  This improves the
documentation but changes none of the results.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43543 0bd85f0b-c151-43b9-a1e9-27231104551f