Fix and improve the documentation of supported groups

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/docs/architecture/tls13-experimental.md b/docs/architecture/tls13-experimental.md
index 5421492..b15d77c 100644
--- a/docs/architecture/tls13-experimental.md
+++ b/docs/architecture/tls13-experimental.md
@@ -135,9 +135,12 @@
 
   (2) The MVP sends one shared secret corresponding to the configured preferred
       group. The preferred group is the group of the first curve in the list of
-      allowed curves as defined by the configuration. By default, it is the
-      mandatory group as defined by section 9.1 of the specification,
-      `secp256r1`. The list of allowed curves can be set through the
+      allowed curves as defined by the configuration. The allowed curves are
+      by default ordered as follow: `secp256r1`, `x25519`, `secp384r1`
+      and finally `secp521r1`. This default order is aligned with the
+      list of mandatory-to-implement groups (in absence of an application
+      profile standard specifying otherwise) defined in section 9.1 of the
+      specification. The list of allowed curves can be changed through the
       `mbedtls_ssl_conf_curves()` API.
 
   (3) The MVP proposes only TLS 1.3 and does not support version negociation.
@@ -148,11 +151,8 @@
       re-initiate a server handshake.
 
 - Supported groups: depends on the library configuration.
-  Minimally (as defined in section 9.1 of the TLS 1.3 specification):
-  secp256r1 and x25519.
-
-  Furthermore, depending on the library configuration, potentially:
-  secp384r1 and secp521r1.
+  Potentially all ECDHE groups but x448:
+  secp256r1, x25519, secp384r1 and secp521r1.
 
   Finite field groups (DHE) are not supported.