DOC: Document the BOOST_FUSION_AUTO* macros.

This documents the AUTO_ADAPT macros :
  * BOOST_FUSION_AUTO_ADAPT_STRUCT
  * BOOST_FUSION_AUTO_ADAPT_STRUCT_NAMED
  * BOOST_FUSION_AUTO_ADAPT_ASSOC_STRUCT
  * BOOST_FUSION_AUTO_ADAPT_ASSOC_STRUCT_NAMED