Minor documentation fix

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index ce0611d..e1f063c 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -101,7 +101,8 @@
  * This macro appearing at the beginning of the declaration of a function
  * indicates that its return value should be generally be checked in portable
  * applications. Omitting the check will result in a compile-time warning if
- * #MBEDTLS_CHECK_RETURN is implemented for the compiler in use.
+ * #MBEDTLS_CHECK_RETURN is implemented for the compiler in use and
+ * #MBEDTLS_CHECK_RETURN_WARNING is enabled in the compile-time configuration.
  *
  * \note  The use of this macro is a work in progress.
  *        This macro will be added to more functions in the future.