[GRASS5] GRASS variables and environment variables

Hi,
   
Sorry for my ignorance, but I have been wondering why we have two kinds
of variables, GRASS and environment things, and when one should be used
over another one. It can be confusing to new users, I think. If there is
a reason, it would be better to document it in lib/init/variables.html.

Thank you.
Huidae Cho