Merged recent changes from trunk.


git-svn-id: http://svn.boost.org/svn/boost/branches/release@86405 b8fc166d-592f-0410-95f2-cb63ce0dd405
coroutine: documentation update


git-svn-id: http://svn.boost.org/svn/boost/branches/release@86407 b8fc166d-592f-0410-95f2-cb63ce0dd405