l3build: some more doc

after joseph's careful alphabetisation with the order of the variables, I've gone and mussed it up again

* separate vars for generated dirs
* I think it is more logical to put ctandir/tdsdir only after distribdir :)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5076 de43f980-851b-0410-b2f7-c40aca1f87e0