Add notes about the purpose and usage of auxiliary header files
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 1935f90..b981f23 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -2,6 +2,16 @@
* \file psa/crypto_struct.h
*
* \brief PSA cryptography module: Mbed TLS structured type implementations
+ *
+ * \note This file may not be included directly. Applications must
+ * include psa/crypto.h.
+ *
+ * This file contains the definitions of some data structures with
+ * implementation-specific definitions.
+ *
+ * In implementations with isolation between the application and the
+ * cryptography module, it is expected that the front-end and the back-end
+ * would have different versions of this file.
*/
/*
* Copyright (C) 2018, ARM Limited, All Rights Reserved