Add notes about the purpose and usage of auxiliary header files
diff --git a/include/psa/crypto_sizes.h b/include/psa/crypto_sizes.h
index f4d2cd8..80b2f9d 100644
--- a/include/psa/crypto_sizes.h
+++ b/include/psa/crypto_sizes.h
@@ -3,12 +3,20 @@
*
* \brief PSA cryptography module: Mbed TLS buffer size macros
*
+ * \note This file may not be included directly. Applications must
+ * include psa/crypto.h.
+ *
* This file contains the definitions of macros that are useful to
* compute buffer sizes. The signatures and semantics of these macros
* are standardized, but the definitions are not, because they depend on
* the available algorithms and, in some cases, on permitted tolerances
* on buffer sizes.
*
+ * 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.
+ *
* Macros that compute sizes whose values do not depend on the
* implementation are in crypto.h.
*/