Document that MBEDTLS_PSA_CRYPTO_CONFIG requires config_psa.h

Document that enabling MBEDTLS_PSA_CRYPTO_CONFIG requires including
mbedtls/config_psa.h from the configuration file (mbedtls/config.h or
MBEDTLS_USER_CONFIG_FILE).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 5fd3c59..98f88ae 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -2057,7 +2057,11 @@
  * API to be configured separately from support through the mbedtls API.
  *
  * Uncomment this to enable use of PSA Crypto configuration settings which
- * can be found in include/psa/crypto_config.h
+ * can be found in include/psa/crypto_config.h.
+ *
+ * If you enable this option and write your own configuration file, you must
+ * include mbedtls/config_psa.h in your configuration file. The default
+ * provided mbedtls/config.h contains the necessary inclusion.
  *
  * This feature is still experimental and is not ready for production since
  * it is not completed.