Document the rule that Python.h must be included before any standard

headers.  This is the final checkin for SF bug #458768.

Doc/api/api.tex

@@ -76,6 +76,9 @@ API are included in your code by the following line:
 This implies inclusion of the following standard headers:
 \code{<stdio.h>}, \code{<string.h>}, \code{<errno.h>},
 \code{<limits.h>}, and \code{<stdlib.h>} (if available).
+Since Python may define some pre-processor definitions which affect
+the standard headers on some systems, you must include \file{Python.h}
+before any standard headers are included.
 
 All user visible names defined by Python.h (except those defined by
 the included standard headers) have one of the prefixes \samp{Py} or