Document undocumented behaviour(s) of the 3D plotting functions

[charpent]

It turns out that our 3D plotting infrastructures has some nice, but undocumented features :

• plot3d and friends accept float("NaN") values as value of a plotted function, and leave the corresponding face(s) unplotted : this gives a rectangular hole un the surface ;

• the same functions accept a2-element list (or tuple ?) [<point function>, <colormap>]as a value for the color keyword argument ; this allows to compute a color value as a function of the point.

As far as I know, this latter point is documented only for parametric surfaces ; the first one is not documented at all.

The point of this ticket is to document these abilities, thus (hopefully) avoiding to lose them in any future upgrade (somewhat needed...) of our 3D infrastructure, and allowing to write new utilities explicitly using them.

[chapoton]

Some documentation for color functions was added after 8.0. Please wait for the doc of 8.1 to see where. This is still very minimal.

[charpent]

We'll wait on a probable upgrade of matplotlib : the undocumented features might change ;-).