Update state of TLS 1.3 functionality in architecture document

Signed-off-by: Hanno Becker <hanno.becker@arm.com>

docs/architecture/tls13-experimental.md

@@ -38,3 +38,12 @@ together with their level of testing:
 - The HKDF key derivation function on which the TLS 1.3 key schedule is based,
   is already present as an independent module controlled by `MBEDTLS_HKDF_C`
   independently of the development of the TLS 1.3 prototype.
+
+- The TLS 1.3-specific HKDF-based key derivation functions (see RFC 8446):
+  * HKDF-Expand-Label
+  * Derive-Secret
+  - Secret evolution
+  * The traffic {Key,IV} generation from secret
+  Those functions are implemented in `library/ssl_tls13_keys.c` and
+  tested in `test_suite_ssl` using test vectors from RFC 8448 and
+  https://tls13.ulfheim.net/.