Fix function variants in l3doc

A previous commit broke the detection of variants, and all functions
appeared twice.  Now fixed: in part, this required using the
\__cs_split_function:NN l3basics internal instead of some other
internals.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5331 de43f980-851b-0410-b2f7-c40aca1f87e0