Graham bemethavinci The actual documentation is http://twig.sensiolabs.org/doc/1.x/templates.html for the version xtgem uses but this just gives basic examples
Xtgem just uses the basic twig limited to the 4 xtgem functions that use twig templates so most of the extension packages and the 'symphony' stuff is not available on xtgem
For the rest it's google for things like 'twig templates tutorial' and searches based on the specific twig command you are researching