Clarify key type support
Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/tf-psa-crypto/include/psa/crypto.h b/tf-psa-crypto/include/psa/crypto.h
index 17a39c3..e5c148d 100644
--- a/tf-psa-crypto/include/psa/crypto.h
+++ b/tf-psa-crypto/include/psa/crypto.h
@@ -5009,6 +5009,8 @@
* The following conditions can result in this error:
* * \p alg is not supported.
* * \p private_key is not supported for use with \p alg.
+ * * Only elliptic curve Diffie-Hellman with ECC keys is supported, not
+ * finite field Diffie_hellman with DH keys.
*
* \retval #PSA_ERROR_INVALID_ARGUMENT \emptydescription
* \retval #PSA_ERROR_INSUFFICIENT_MEMORY \emptydescription