The .multichoice:nn key properties is useful, so remove 'experimental' status


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4523 de43f980-851b-0410-b2f7-c40aca1f87e0
Add key-choice-unknown message text

Also move one message which may yet get deprecated with the code it
works with.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4524 de43f980-851b-0410-b2f7-c40aca1f87e0