Document _FloatN and _FloatNx versions of math functions

Document _FloatN and _FloatNx versions of math functions

The functions defined in ISO/IEC TS 18661-3 take floating-point arguments
and return floating-point numbers of _FloatN and _FloatNx types.  Apart
from the type, these functions behave the same as their float, double, and
long double counterparts.  This patch adds the newer functions to the
manual.

	* manual/arith.texi (Infinity and NaN): Document SNANFN and SNANFNx.
	(Error Reporting by Mathematical Functions): Document HUGE_VAL_FN
	and HUGE_VAL_FNx.
	(Absolute Value): Document fabsfN, fabsfNx, cabsfN, cabsfNx.
	Rephrase the paragraph that mentions that fabs, fabsf, and fabsl
	are in math.h, to avoid having to list the _FloatN and _FloatNx
	variants as well.  Likewise for the cabs functions.
	(Normalization Functions): Document frexpfN, frexpfNx, ldexpfN,
	ldexpfNx, scalbnfN, scalbnfNx, scalblnfN, scalblnfNx.
	Mention that _FloatN and _FloatNx variants of scalbn and scalbln
	come from TS 18661-3, since this section explicitly states that
	these functions come from BSD.
	(Rounding Functions): Document ceilfN, ceilfNx, floorfN, floorfNx,
	truncfN, truncfNx, rintfN, rintfNx, nearbyintfN, nearbyintfNx,
	roundfN, roundfNx, roundevenfN, roundevenfNx, lrintfN, lrintfNx,
	llrintfN, llrintfNx, lroundfN, lroundfNx, llroundfN, llroundfNx,
	fromfpfN, fromfpfNx, ufromfpfN, ufromfpfNx, fromfpxfN, fromfpxfNx,
	ufromfpxfN, ufromfpxfNx, modffN, modffNx.
	(Remainder Functions): Document fmodfN, fmodfNx, remainderfN,
	remainderfNx.
	(Setting and modifying single bits of FP values): Document
	copysignfN, copysignfNx, nextafterfN, nextafterfNx, nextupfN,
	nextupfNx, nextdownfN, nextdownfNx, nanfN, nanfNx, canonicalizefN,
	canonicalizefNx, getpayloadfN, getpayloadfNx, setpayloadfN,
	setpayloadfNx, setpayloadsigfN, setpayloadsigfNx.
	(Floating-Point Comparison Functions): Document totalorderfN,
	totalorderfNx, totalordermagfN, totalordermagfNx.
	(Miscellaneous FP arithmetic functions): Document fminfN, fminfNx,
	fmaxfN, fmaxfNx, fminmagfN, fminmagfNx, fmaxmagfN, fmaxmagfNx,
	fdimfN, fdimfNx, fmafN, fmafNx.
	(Complex Numbers): Document the complex types: _FloatN complex and
	_FloatNx complex.
	(rojections, Conjugates, and Decomposing of Complex Numbers):
	Document crealfN, crealfNx, cimagfN, cimagfNx, conjfN, conjfNx,
	cargfN, cargfNx, cprojfN, cprojfNx.
	* manual/math.texi (Mathematics): Mention that the _FloatN and
	_FloatNx variants of the math functions come from TS 18661-3,
	unless otherwise stated.
	(Predefined Mathematical Constants): Document the _FloatN and
	_FloatNx variants of the macros prefixed with M_.
	(Trigonometric Functions): Document sinfN, sinfNx, cosfN, cosfNx,
	tanfN, tanfNx, sincosfN, sincosfNx, csinfN, csinfNx, ccosfN,
	ccosfNx, ctanfN, ctanfNx.
	(Inverse Trigonometric Functions): Document asinfN, asinfNx,
	acosfN, acosfNx, atanfN, atanfNx, atan2fN, atan2fNx.
	(Exponentiation and Logarithms): Document expfN, expfNx, exp2fN,
	exp2fNx, exp10fN, exp10fNx, logfN, logfNx, log10fN, log10fNx,
	log2fN, log2fNx, logbfN, logbfNx, ilogbfN, ilogbfNx, llogbfN,
	llogbfNx, powfN, powfNx, sqrtfN, sqrtfNx, cbrtfN, cbrtfNx, hypotfN,
	hypotfNx, expm1fN, expm1fNx, log1pfN, log1pfNx, cexpfN, cexpfNx,
	clogfN, clogfNx, clog10fN, clog10fNx, csqrtfN, csqrtfNx, cpowfN,
	cpowfNx.
	(Hyperbolic Functions): sinhfN, sinhfNx, coshfN, coshfNx, tanhfN,
	tanhfNx, csinhfN, csinhfNx, ccoshfN, ccoshfNx, ctanhfN, ctanhfNx,
	asinhfN, asinhfNx, acoshfN, acoshfNx, atanhfN, atanhfNx, casinhfN,
	casinhfNx, cacoshfN, cacoshfNx, catanhfN, catanhfNx.
	(Special Functions): Document erffN, erffNx, erfcfN, erfcfNx,
	lgammafN, lgammafNx, lgammarfN_r, lgammafNx_r, tgammafN, tgammafNx,
	j0fN, j0fNx, j1fN, j1fNx, jnfN, jnfNx, y0fN, y0fNx, y1fN, y1fNx,
	ynfN, ynfNx.