Add documentation for all Lua vars in l3build

This covers all of the ones anyone should be changing and gives hopefully
some clue as to the point of them.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5071 de43f980-851b-0410-b2f7-c40aca1f87e0
Target "cmdcheck" is standard but only logical if using l3doc

By default, this is not enabled so doesn't show up in e.g. help.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5072 de43f980-851b-0410-b2f7-c40aca1f87e0
Tighten up a bit what a bundle is


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5073 de43f980-851b-0410-b2f7-c40aca1f87e0