Okay Stratis, please give the attached product a try and let me know what you think.
There are 2 phrases:
The first one is what displays under the edit reason field, and the second one is the message shown to users who try to submit an edit without giving a reason.
In the product settings, you can enable/disable the product (you will have to initially enable it), and you can select usergroups who will be exempt from having to give a reason for editing posts.