add documentation

Reviewed-by: NRich Salz <rsalz@openssl.org>

doc/apps/genpkey.pod

@@ -192,6 +192,9 @@ numeric OID. Following parameter sets are supported:
 
 =back
 
+=head1 X25519 KEY GENERATION OPTIONS
+
+The X25519 algorithm does not currently support any key generation options.
 
 
 =head1 NOTES
@@ -253,6 +256,10 @@ Generate EC key directly:
         -pkeyopt ec_paramgen_curve:P-384 \
         -pkeyopt ec_param_enc:named_curve
 
+Generate an X25519 private key:
+
+ openssl genpkey -algorithm X25519 -out xkey.pem
+
 =head1 HISTORY
 
 The ability to use NIST curve names, and to generate an EC key directly,

doc/apps/pkeyutl.pod

@@ -240,6 +240,11 @@ verify operations use ECDSA and derive uses ECDH. Currently there are no
 additional options other than B<digest>. Only the SHA1 digest can be used and
 this digest is assumed by default.
 
+=head1 X25519 ALGORITHM
+
+The X25519 algorithm supports key derivation only. Currently there are no
+additional options.
+
 =head1 EXAMPLES
 
 Sign some data using a private key: