Document "Make a copy" link

Commentary from issue2550674.

Author: rouilj

doc/user_guide.txt

@@ -305,6 +305,9 @@ and messages, and the issue history at the bottom of the page:
 
 .. image:: images/edit_issue.png
 
+The "Make a copy" link duplicate the current issue copying most of the
+properties to a new issue. This is useful for splitting am issue into
+sub-issues that can be assigned to different people.
 
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