Add notes about the purpose and usage of auxiliary header files
diff --git a/include/psa/crypto_platform.h b/include/psa/crypto_platform.h
index 7aabd1b..9af320d 100644
--- a/include/psa/crypto_platform.h
+++ b/include/psa/crypto_platform.h
@@ -2,6 +2,16 @@
* \file psa/crypto_platform.h
*
* \brief PSA cryptography module: Mbed TLS platfom definitions
+ *
+ * \note This file may not be included directly. Applications must
+ * include psa/crypto.h.
+ *
+ * This file contains platform-dependent type definitions.
+ *
+ * In implementations with isolation between the application and the
+ * cryptography module, implementers should take care to ensure that
+ * the definitions that are exposed to applications match what the
+ * module implements.
*/
/*
* Copyright (C) 2018, ARM Limited, All Rights Reserved