Merged bug fixes and doc updates from bleeding-edge. Should resolve compilation errors with MSVC.

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@812 ab6a8de6-4e00-4c3d-a860-8899c17d353f
Fixed a race condition that makes possible for flush() to conflict with the dedicated feeding thread if it fails to enter run() before flush().

git-svn-id: https://boost-log.svn.sourceforge.net/svnroot/boost-log/trunk/boost-log@814 ab6a8de6-4e00-4c3d-a860-8899c17d353f