l3build: minus a proof-read, i'm happy with the doc

* twocolumn ToC looks better
* describe better some variables in the text that were only listed in the big table 
* add brief description of all \START, \END, etc., commands

I think we've basically covered everything now

N.B. my todo on a possible \cs{TRACEPAGES}; any thoughts?
     especially w.r.t. it being active by default?

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5130 de43f980-851b-0410-b2f7-c40aca1f87e0