[Rd] Incorrect comment about ISNA(x) in Arith.h (PR#13826)

[rocket at google.com]

R-2.9.0/include/R_ext/Arith.h has:

int R_IsNA(double);		/* True for R's NA only */
int R_IsNaN(double);		/* True for special NaN, *not* for NA */
int R_finite(double);		/* True if none of NA, NaN, +/-Inf */

#define ISNA(x)	       R_IsNA(x)
/* True for *both* NA and NaN.

The first and last lines are contradictory - if R_IsNA is true only
for NA, not NaN, then ISNA should be the same.

Venables & Ripley S Programming p 137 indicates that
"ISNA does not detect NaN s"

Here's a little .c file and R transcript that shows that V&R are correct,
   ISNA(x) is not True for NaN
So the last comment should read
/* True for NA, not NaN



#include "R.h"
void mysum(double *x)
{
  double sum = 0.0;
  sum = x[0] + x[1] + x[2];
  if (ISNA(sum))
    PROBLEM "sum is NA" WARNING(NULL_ENTRY);
  if (ISNAN(sum))
    PROBLEM "sum is NAN" WARNING(NULL_ENTRY);
  if (!(R_FINITE(sum)))
    PROBLEM "sum is not finite" WARNING(NULL_ENTRY);
  return;
}


> dyn.load("~/R/work/bug.so")
> .C("mysum", c(NaN, 0.0, 0.0), NAOK = TRUE, DUP = FALSE)
[[1]]
[1] NaN   0   0

Warning messages:
1: sum is NAN 
2: sum is not finite 



--please do not edit the information below--

Version:
 platform = x86_64-unknown-linux-gnu
 arch = x86_64
 os = linux-gnu
 system = x86_64, linux-gnu
 status = 
 major = 2
 minor = 9.0
 year = 2009
 month = 04
 day = 17
 svn rev = 48333
 language = R
 version.string = R version 2.9.0 (2009-04-17)

Locale:
LC_CTYPE=en_US;LC_NUMERIC=C;LC_TIME=C;LC_COLLATE=C;LC_MONETARY=C;LC_MESSAGES=en_US;LC_PAPER=en_US;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US;LC_IDENTIFICATION=C

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