doc: give examples of command to create key/dh files

docs/re6stnet.rst

@@ -116,6 +116,8 @@ certificates, as follows: translate the significant part to hexadecimal
   openssl req -nodes -new -x509 -key ca.key -set_serial 0x120010db80042 \
               -days 365 -out ca.crt
 
+(see ``re6st-registry --help`` for examples to create key/dh files)
+
 The CA email will be used as sender for mails containing tokens.
 The registry can now be started::
 

re6st/cli/registry.py

@@ -72,10 +72,13 @@ def main():
         help="Path to SQLite database file. It is automatically initialized"
              " if the file does not exist.")
     _('--dh',
-        help='File containing Diffie-Hellman parameters in .pem format')
+        help="File containing Diffie-Hellman parameters in .pem format."
+             " To generate them, you can use something like:\n"
+             "openssl dhparam -out dh2048.pem 2048")
     _('--ca', required=True, help=parser._ca_help)
     _('--key', required=True,
-            help="CA private key in .pem format.")
+            help="CA private key in .pem format. For example:\nopenssl"
+            " genpkey -out ca.key -algorithm rsa -pkeyopt rsa_keygen_bits:2048")
     _('--mailhost', required=True,
             help="SMTP host to send confirmation emails. For debugging"
                  " purpose, it can also be an absolute or existing path to"