RT1596: Add clarifying doc.

Reviewed-by: NViktor Dukhovni <viktor@openssl.org>

doc/apps/x509.pod

@@ -332,7 +332,8 @@ subject name (i.e.  makes it self signed) changes the public key to the
 supplied value and changes the start and end dates. The start date is
 set to the current time and the end date is set to a value determined
 by the B<-days> option. Any certificate extensions are retained unless
-the B<-clrext> option is supplied.
+the B<-clrext> option is supplied; this includes, for example, any existing
+key identifier extensions.
 
 If the input is a certificate request then a self signed certificate
 is created using the supplied private key using the subject name in