Document max_iter parameter correctly, see: https://svn.boost.org/trac/boost/ticket/9225
Add and document options to enable/disable __float128 usage explicitly, see: https://svn.boost.org/trac/boost/ticket/9240
Clean up negative argument usage in Bessel I0 and I1: negative arguments are handled at a higher level, so strip out support for them here and add an assert.  See https://svn.boost.org/trac/boost/ticket/9512.
Update history and regenerate docs.