Add a note about CTR_DRBG security strength to config.h
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 085f40a..834cced 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -2158,6 +2158,10 @@
* The CTR_DRBG generator uses AES-256 by default.
* To use AES-128 instead, enable \c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY above.
*
+ * \note To achieve a 256-bit security strength with CTR_DRBG,
+ * you must use AES-256 *and* use sufficient entropy.
+ * See ctr_drbg.h for more details.
+ *
* Module: library/ctr_drbg.c
* Caller:
*