Merged changes from bleeding-edge.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@862 ab6a8de6-4e00-4c3d-a860-8899c17d353f
Documentation fixes.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@866 ab6a8de6-4e00-4c3d-a860-8899c17d353f