Document nesting for \keys_set_known:nnN

As the defined behaviour has altered (or at least been clarified), I've
set an 'updated' date. I guess people like Will need this information!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4707 de43f980-851b-0410-b2f7-c40aca1f87e0