RT1596: Add clarifying doc.

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

doc/apps/x509.pod

diff --git a/doc/apps/x509.pod b/doc/apps/x509.pod
index e4bcb4a..7dde927 100644
--- a/doc/apps/x509.pod
+++ b/doc/apps/x509.pod
@@ -332,7 +332,8 @@
 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