Added sentence about why 'edited just now' appears under the converted comment

doc/user/discussions/index.md

@@ -354,7 +354,8 @@ Clicking on the **Reply to comment** button will bring the reply area into focus
 ![Reply to comment feature](img/reply_to_comment.gif)
 
 Relying to a non-discussion comment will convert the non-discussion comment to a 
-threaded discussion once the reply is submitted.
+threaded discussion once the reply is submitted. This conversion is considered an edit
+to the original comment, so a note about when it was last edited will appear underneath it.
 
 This feature only exists for Issues, Merge requests, and Epics. Commits, Snippets and Merge request diff discussions are not supported yet.