Update limitation of psk_client_cb and psk_server_cb in usage with TLSv1.3

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/11816)

doc/man3/SSL_CTX_set_psk_client_callback.pod

@@ -123,7 +123,9 @@ and it will use that in preference. If no such callback is present then it will
 check to see if a callback has been set via SSL_CTX_set_psk_client_callback() or
 SSL_set_psk_client_callback() and use that. In this case the B<hint> value will
 always be NULL and the handshake digest will default to SHA-256 for any returned
-PSK.
+PSK. TLSv1.3 early data exchanges are possible in PSK connections only with the
+B<SSL_psk_use_session_cb_func> callback, and are not possible with the
+B<SSL_psk_client_cb_func> callback.
 
 =head1 NOTES
 

doc/man3/SSL_CTX_use_psk_identity_hint.pod

@@ -83,7 +83,9 @@ via SSL_CTX_set_psk_find_session_callback() or SSL_set_psk_find_session_callback
 and it will use that in preference. If no such callback is present then it will
 check to see if a callback has been set via SSL_CTX_set_psk_server_callback() or
 SSL_set_psk_server_callback() and use that. In this case the handshake digest
-will default to SHA-256 for any returned PSK.
+will default to SHA-256 for any returned PSK. TLSv1.3 early data exchanges are
+possible in PSK connections only with the B<SSL_psk_find_session_cb_func>
+callback, and are not possible with the B<SSL_psk_server_cb_func> callback.
 
 A connection established via a TLSv1.3 PSK will appear as if session resumption
 has occurred so that L<SSL_session_reused(3)> will return true.