[manual] suggested addition to pcolor description

[John W. Eaton]

On 20-Jan-2009, Ben Abbott wrote:

| On caveat is that gnuplot's method of interpolation is different
| than that of Matlab's. As Francesco points out the commercial
| variant splits rectangles into a pair of triangles and performs a
| linear interpolation of the color across the triangle. Although I do
| not know what algorithm is used, gnuplot does not work the same way.

Is this something that Octave should do, rather than relying on
gnuplot?  If so, where does this code belong?  Maybe we should provide
a function (accessible from C++ and the scripting language?) that does
the interpolation, so that all backends can do things consistently.

jwe