To use the "Reason for Edit" hack/feature, simply go into a post, click on the EDIT button, and choose FULL EDIT.
Before you click the 'submit modified post' button, to save/post any changes to a topic/reply, just below the add attachments, you'll see:Edit Options:
Add the 'Edit by' line in this post?
Reason for editing post (optional)
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Number of downloads: 32
I have it set NOT to include the edits automatically - so you need to check off the 'edit by' tick box and then if you choose to, add a reason for edit in the entry box below.
See my attachment for an image.
P.S. Once you've read this, I will move it to the FAQ forum.