copy: document current behavior of 'hg cp --after'

I'm about to propose an output change here, but the existing behavior
was untested!

tests/test-copy.t

@@ -224,6 +224,9 b' copy --after on an added file'
 foo was clean:
   $ hg st -AC foo
   C foo
+Trying to copy on top of an existing file fails,
+  $ hg copy -A bar foo
+  foo: not overwriting - file exists
 but it's considered modified after a copy --after --force
   $ hg copy -Af bar foo
   $ hg st -AC foo