Document what "TLS 1.3 depends on PSA" entails

Explicitly document that when using TLS 1.3, you must initialize PSA crypto
before starting a handshake.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 4b954bb..2594964 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -4684,6 +4684,11 @@
  *                 in which case the datagram of the underlying transport that is
  *                 currently being processed might or might not contain further
  *                 DTLS records.
+ *
+ * \note           If the context is configured to allow TLS 1.3, or if
+ *                 #MBEDTLS_USE_PSA_CRYPTO is enabled, the PSA crypto
+ *                 subsystem must have been initialized by calling
+ *                 psa_crypto_init() before calling this function.
  */
 int mbedtls_ssl_handshake(mbedtls_ssl_context *ssl);