Document sNaN argument error handling.

TS 18661-1 says that "Whether a signaling NaN input causes a domain
error is implementation-defined.".  Considering it a domain error
would (given glibc's math_errhandling definition) mean setting errno
to EDOM.  glibc consistently does not set errno for sNaN inputs
(unless it does so for qNaN as well, i.e. iseqsig), so this patch adds
documentation of the implementation-defined choice not to treat this
case as a domain error.

	* manual/arith.texi (Math Error Reporting): Document that sNaN
	arguments are not considered domain errors.