Add BOOST_EXPLICIT_OPERATOR_BOOL documentation.

git-svn-id: http://svn.boost.org/svn/boost/trunk@86739 b8fc166d-592f-0410-95f2-cb63ce0dd405
Updated docs for is_sorted, etc. Refs #9367

git-svn-id: http://svn.boost.org/svn/boost/trunk@86741 b8fc166d-592f-0410-95f2-cb63ce0dd405