* manual/math.texi (Mathematics): Add description for _FloatN and
_FloatNx types and mentions that they are not support in glibc for
any architecture, so far.
* manual/arith.texi (wcstof, wcstold): Replace the mention to
stdlib.h with wchar.h.
(Parsing of Floats): Add descriptions for strtofN and wcstofN.
(Printing of Floats): Add description for strfromfN.