Improve docs for mbedtls_zeroize() and add refs
diff --git a/include/mbedtls/utils.h b/include/mbedtls/utils.h
index 61b1b76..cb03fb0 100644
--- a/include/mbedtls/utils.h
+++ b/include/mbedtls/utils.h
@@ -33,6 +33,13 @@
*
* \note This implementation should never be optimized out by the
* compiler
+ *
+ * \note It is extremely difficult to guarantee that calls to
+ * mbedtls_zeroize() are not removed by aggressive compiler
+ * optimizations in a portable way. For this reason, Mbed TLS
+ * provides the configuration option MBEDTLS_UTILS_ZEROIZE_ALT,
+ * which allows users to configure mbedtls_zeroize() to use a
+ * suitable implementation for their platform and needs
*/
void mbedtls_zeroize( void *buf, size_t len );