Add a note in the docs about sharing PSKs between TLSv1.2 and TLSv1.3

Fixes #6490
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NPaul Yang <yang.yang@baishancloud.com>
(Merged from https://github.com/openssl/openssl/pull/7044)

doc/man3/SSL_CTX_set_psk_client_callback.pod

@@ -132,6 +132,14 @@ Note that parameter B<hint> given to the callback may be B<NULL>.
 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.
 
+There are no known security issues with sharing the same PSK between TLSv1.2 (or
+below) and TLSv1.3. However the RFC has this note of caution:
+
+"While there is no known way in which the same PSK might produce related output
+in both versions, only limited analysis has been done.  Implementations can
+ensure safety from cross-protocol related output by not reusing PSKs between
+TLS 1.3 and TLS 1.2."
+
 =head1 RETURN VALUES
 
 Return values from the B<SSL_psk_client_cb_func> callback are interpreted as

doc/man3/SSL_CTX_use_psk_identity_hint.pod

@@ -123,6 +123,16 @@ completely.
 The B<SSL_psk_find_session_cb_func> callback should return 1 on success or 0 on
 failure. In the event of failure the connection setup fails.
 
+=head1 NOTES
+
+There are no known security issues with sharing the same PSK between TLSv1.2 (or
+below) and TLSv1.3. However the RFC has this note of caution:
+
+"While there is no known way in which the same PSK might produce related output
+in both versions, only limited analysis has been done.  Implementations can
+ensure safety from cross-protocol related output by not reusing PSKs between
+TLS 1.3 and TLS 1.2."
+
 =head1 SEE ALSO
 
 L<SSL_CTX_set_psk_use_session_callback(3)>,