Update docs of Boost.Variant. Add advice about recursive_wrapper performance (refs #7718)

git-svn-id: http://svn.boost.org/svn/boost/trunk@84104 b8fc166d-592f-0410-95f2-cb63ce0dd405
Add tentative support for user-defined-literals.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84106 b8fc166d-592f-0410-95f2-cb63ce0dd405