Document deprecated transaction system as non thread safe

Not all of the writes to this field are protected by a mutex.
There is no also no protection in place to stop another thread from overwriting
the current transaction

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto_storage.h b/library/psa_crypto_storage.h
index f1ea265..d7f5b18 100644
--- a/library/psa_crypto_storage.h
+++ b/library/psa_crypto_storage.h
@@ -231,8 +231,9 @@
  * This type is designed to be serialized by writing the memory representation
  * and reading it back on the same device.
  *
- * \note The transaction mechanism is designed for a single active transaction
- *       at a time. The transaction object is #psa_crypto_transaction.
+ * \note The transaction mechanism is not thread-safe. There can only be one
+ *       single active transaction at a time.
+ *       The transaction object is #psa_crypto_transaction.
  *
  * \note If an API call starts a transaction, it must complete this transaction
  *       before returning to the application.