Thread: make barrier not copyable.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84064 b8fc166d-592f-0410-95f2-cb63ce0dd405
Thread: fix some typos in externally_locked doc.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84065 b8fc166d-592f-0410-95f2-cb63ce0dd405