Document default client -psk_identity

Document that -psk is required to use PSK cipher
[skip ci]
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/3607)

doc/man1/s_client.pod

@@ -378,12 +378,14 @@ Can be used to override the implicit B<-ign_eof> after B<-quiet>.
 =item B<-psk_identity identity>
 
 Use the PSK identity B<identity> when using a PSK cipher suite.
+The default value is "Client_identity" (without the quotes).
 
 =item B<-psk key>
 
 Use the PSK key B<key> when using a PSK cipher suite. The key is
 given as a hexadecimal number without leading 0x, for example -psk
 1a2b3c4d.
+This option must be provided in order to use a PSK cipher.
 
 =item B<-ssl3>, B<-tls1>, B<-tls1_1>, B<-tls1_2>, B<-tls1_3>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3>
 

doc/man1/s_server.pod

@@ -344,6 +344,7 @@ identity is the string "Client_identity".
 Use the PSK key B<key> when using a PSK cipher suite. The key is
 given as a hexadecimal number without leading 0x, for example -psk
 1a2b3c4d.
+This option must be provided in order to use a PSK cipher.
 
 =item B<-ssl2>, B<-ssl3>, B<-tls1>, B<-tls1_1>, B<-tls1_2>, B<-tls1_3>, B<-no_ssl2>, B<-no_ssl3>, B<-no_tls1>, B<-no_tls1_1>, B<-no_tls1_2>, B<-no_tls1_3>