Clarify what SSL_get_session() does on the server side in TLSv1.3

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16582)

doc/man3/SSL_get_session.pod

@@ -37,8 +37,11 @@ L<SSL_SESSION_is_resumable(3)> for information on how to determine whether an
 SSL_SESSION object can be used for resumption or not.
 
 Additionally, in TLSv1.3, a server can send multiple messages that establish a
-session for a single connection. In that case the above functions will only
-return information on the last session that was received.
+session for a single connection. In that case, on the client side, the above
+functions will only return information on the last session that was received. On
+the server side they will only return information on the last session that was
+sent, or if no session tickets were sent then the session for the current
+connection.
 
 The preferred way for applications to obtain a resumable SSL_SESSION object is
 to use a new session callback as described in L<SSL_CTX_sess_set_new_cb(3)>.