#87 docs of z axis default vector

[Kevin Ryde]

It'd be good if the manual said a little more about the nature of the default z axis vector.

I thought to use some "rotate around" etc to draw in 3D, and then set a viewing angle with further overall "rotate around"s. If I'm not mistaken the default z is like a shear, whereas when rotating coordinates a z=0,0 for perpendicular to the page would be wanted. Is that right? The manual could give some guidance if so or if not.

As a concrete suggestion perhaps something like below for section "25.2 The XY- and XYZ-Coordinate Systems" item "/tikz/z". Or a cross reference if this already exists but I did not find it:

The default z=(3.85mm,-3.85mm) behaves as a shear to make the z part of coordinates visible when projected onto the flat page.

z=(0,0) gives a z axis pointing directly out of the page. This is good if you're applying your own rotations to construct and then view a 3-D object from some angle. (View in the sense of project the rotated parts onto the page as the XY plane.)