l3build: try (?) and improve \END documentation

I still half-wonder if \END should be set up in LaTeX to
run \end{document} internally, with \OMIT used to hide
any unwanted material.

Then we'd just say "use |\END| to end the test, always"

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5135 de43f980-851b-0410-b2f7-c40aca1f87e0