Strip exactly one set of braces if present inside optional arguments

This is what LaTeX2e does, but is done here in a very carefully controlled
way (so that the other ideas we have in xparse work).

Note that this may well be a rather brute-force approach to solving the issue:
there is probably a better way to implement all of this!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4462 de43f980-851b-0410-b2f7-c40aca1f87e0
Remove erroneous info in xparse documentation


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4463 de43f980-851b-0410-b2f7-c40aca1f87e0
Document new brace-stripping approach in xparse


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4464 de43f980-851b-0410-b2f7-c40aca1f87e0