Document TLS 1.3 MVP limitation regarding MBEDTLS_USE_PSA_CRYPTO

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/docs/architecture/tls13-support.md b/docs/architecture/tls13-support.md
index 9e9aad3..722ac63 100644
--- a/docs/architecture/tls13-support.md
+++ b/docs/architecture/tls13-support.md
@@ -126,8 +126,11 @@
 
   The TLS 1.3 MVP is compatible with all TLS 1.2 configuration options in the
   sense that when enabling the TLS 1.3 MVP in the library there is no need to
-  modify the configuration for TLS 1.2. Mbed TLS SSL/TLS related features are
-  not supported or not applicable to the TLS 1.3 MVP:
+  modify the configuration for TLS 1.2. The MBEDTLS_USE_PSA_CRYPTO configuration
+  option is an exception though, the TLS 1.3 MVP is not compatible with it.
+
+  Mbed TLS SSL/TLS related features are not supported or not applicable to the
+  TLS 1.3 MVP:
 
   | Mbed TLS configuration option            | Support |
   | ---------------------------------------- | ------- |
@@ -161,7 +164,7 @@
   | MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED    | n/a     |
   | MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED     | n/a     |
   |                                          |         |
-  | MBEDTLS_USE_PSA_CRYPTO                   | no      |
+  | MBEDTLS_USE_PSA_CRYPTO                   | no  (3) |
 
   (1) Some support has already been upstreamed but it is incomplete.
   (2) Key exchange configuration options for TLS 1.3 will likely to be