Adding documentation of the new ADAPT_STRUCT and ADAPT_STRUCT_TPL macros,
capable of deducing type.
Removing AUTO_ADAPT_STRUCT_NAMED, AUTO_ADAPT_ASSOC_STRUCT and AUTO_ADAPT_ASSOC_STRUCT_NAMED.

As discussed with Agustín Bergé and Joel de Guzman on https://github.com/boostorg/fusion/pull/3,
it's better to remove the separate macro for the type deducting macros, and
to instead support a flexible API allowing backward compatibility and type
omission.

This is what ADAPT_STRUCT and ADAPT_STRUCT_TPL now provides. All other
macros will be improved in the same way in the following commits.