Doco for VGshifted added into user.tex.

Fixes #3393

This document had fallen a few weeks behind the code
so was substantially revised to with my current thoughts.
Merge pull request #3395 from WilkAndy/vgshifted_doco_3393

Doco for VGshifted added into user.tex.
Merge commit '8bf28ba7f5ca53ae82a36c35754bc1dcb0f0198c'