Make the variable environment detect whether to be macro/function

Instead of redefining the variable environment inside the documentation
environment and inside the implementation environment, use a boolean to
denote whether we are in the implementation, and let the variable
environment decide for itself.  Also remove the variabledoc environment.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5326 de43f980-851b-0410-b2f7-c40aca1f87e0