float128: Add strfromf128, strtof128, and wcstof128 to the manual

	* 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.