merge: mark ancient debugging option

mercurial/merge.py

@@ -1070,6 +1070,7 def update(repo, node, branchmerge, forc
                     # Allow jumping branches if clean and specific rev given
                     pas = [p1]
 
+        # deprecated config: merge.followcopies
         followcopies = False
         if overwrite:
             pas = [wc]