[manual] suggested addition to pcolor description

[Petr Mikulik]

> | Using "interpolate 0,0" the result renders very rapidly.
> | 
> | Petr, Thanks for pointing out my err, the result looks very good to  
> | me. The corrected changeset is attached ... attributed to you of  
> | course ;-)
> 
> I applied it.

I have also applied the gnuplot patch to its cvs. So "interp shading" should 
work nicely.

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Petr Mikulik