Doc: improve commentary about providing our own definitions of M_PI.

contrib/earthdistance/earthdistance.c

@@ -6,6 +6,7 @@
 
 #include "utils/geo_decls.h"	/* for Point */
 
+/* X/Open (XSI) requires <math.h> to provide M_PI, but core POSIX does not */
 #ifndef M_PI
 #define M_PI 3.14159265358979323846
 #endif

src/bin/pgbench/pgbench.c

@@ -68,6 +68,7 @@
 #include "port/pg_bitutils.h"
 #include "portability/instr_time.h"
 
+/* X/Open (XSI) requires <math.h> to provide M_PI, but core POSIX does not */
 #ifndef M_PI
 #define M_PI 3.14159265358979323846
 #endif

src/include/utils/float.h

@@ -17,8 +17,8 @@
 
 #include <math.h>
 
+/* X/Open (XSI) requires <math.h> to provide M_PI, but core POSIX does not */
 #ifndef M_PI
-/* From my RH5.2 gcc math.h file - thomas 2000-04-03 */
 #define M_PI 3.14159265358979323846
 #endif