mbedtls_config: extend documentation for MBEDTLS_PSA_CRYPTO_C

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/include/mbedtls/mbedtls_config.h b/include/mbedtls/mbedtls_config.h
index 94e21e2..5670d67 100644
--- a/include/mbedtls/mbedtls_config.h
+++ b/include/mbedtls/mbedtls_config.h
@@ -3167,7 +3167,8 @@
  *
  * Requires: either MBEDTLS_CTR_DRBG_C and MBEDTLS_ENTROPY_C,
  *           or MBEDTLS_HMAC_DRBG_C and MBEDTLS_ENTROPY_C,
- *           or MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG.
+ *           or MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG;
+ *           MBEDTLS_CIPHER_C if any unauthenticated cipher is builtin in PSA.
  *
  */
 #define MBEDTLS_PSA_CRYPTO_C