Improve comment for att_isnull.

The comment implies that a 1 in the null bitmap indicates a null value,
but actually a 0 in the null bitmap indicates a null value. Try to
be more clear.

Patch by me; proposed wording reviewed by Alvaro Herrera and Tom Lane.

Discussion: http://postgr.es/m/CA+TgmobHOP8r6cG+UnsDFMrS30-m=jRrCBhgw-nFkn0k9QnFsg@mail.gmail.com

src/include/access/tupmacs.h

@@ -16,7 +16,9 @@
 
 
 /*
- * check to see if the ATT'th bit of an array of 8-bit bytes is set.
+ * Check a tuple's null bitmap to determine whether the attribute is null.
+ * Note that a 0 in the null bitmap indicates a null, while 1 indicates
+ * non-null.
  */
 #define att_isnull(ATT, BITS) (!((BITS)[(ATT) >> 3] & (1 << ((ATT) & 0x07))))