few more docs

Hi,

although I might touch these sources again tomorrow, I thought it's good to open a PR in order to receive early feedback (mostly of crude type about missing pieces/paragraphs). Yet, any feedback is welcome :)

tia, F.
use `$MODULEPATH`, and correct to: "and $MODULEPATH should be aligned with it to make sure the EasyBuild module can be loaded, i.e. you should include <prefix>/modules/all in your $MODULEPATH."