manual: Complete @standards in arith.texi.

	* manual/arith.texi (FP_NAN): Add or complete header and
	standard annotations.
	(FP_INFINITE): Likewise.
	(FP_ZERO): Likewise.
	(FP_SUBNORMAL): Likewise.
	(FP_NORMAL): Likewise.
	(SNAN): Likewise.
	(SNANL): Likewise.
	(totalorderf): Likewise.
	(totalorderl): Likewise.
	(totalordermagf): Likewise.
	(totalordermagl): Likewise.
	(_Complex_I): Likewise.
	(I): Likewise.