Change:

    MX   This margin code is used to denote IEC 60559:1989 standard
         floating-point extensions.

to:

    MX and MXX  These two margin codes both relate to the IEC 60559
                Floating-Point option.  The MX code denotes
                functionality that is mandated by the ISO C standard
                for IEC 60559 implementations; the MXX code denotes
                IEC 60559 functionality that is an extension to the
                ISO C standard.