Greetings Murad,
Unfortunately the changes in the long text will not be displayed in the change log/action log. The purpose of the timestamp in the long text itself is to show when the changes were made and by whom.
You could look into a development using the user exit QQMA0014 to for example set a user status like LTUP - "Long Text Updated" to designate that a Notification long text was changed. A status change would then appear in the action log.