Document -force_pubkey option.

doc/apps/x509.pod

@@ -48,6 +48,7 @@ B<openssl> B<x509>
 [B<-CAkey filename>]
 [B<-CAcreateserial>]
 [B<-CAserial filename>]
+[B<-force_pubkey key>]
 [B<-text>]
 [B<-C>]
 [B<-md2|-md5|-sha1|-mdc2>]
@@ -400,6 +401,15 @@ specified then the extensions should either be contained in the unnamed
 L<x509v3_config(5)|x509v3_config(5)> manual page for details of the
 extension section format.
 
+=item B<-force_pubkey key>
+
+when a certificate is created set its public key to B<key> instead of the
+key in the certificate or certificate request. This option is useful for
+creating certificates where the algorithm can't normally sign requests, for
+example DH.
+
+The format or B<key> can be specified using the B<-keyform> option.
+
 =back
 
 =head2 NAME OPTIONS