Mention the crash risk without runtime detection

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 5474060..6fcd025 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -4015,6 +4015,7 @@
  * detection will be used to select between them.
  *
  * If only one implementation is present, runtime detection will not be used.
+ * This configuration will crash if running on the CPU without needed features.
  */
 //#define MBEDTLS_AES_USE_HARDWARE_ONLY