record - also mention how to save without sending

In (info "(mailutils) Mail Variables")

'record'

     Type: String.
     Default: Unset.

     When set, any outgoing message will be saved to the named file.
Add:           ^a copy of

Also mention how to save without also actually sending. So one can see
what would have been sent, without sending.

E.g., "change the 'sendmail' variable to /bin/cat ..."