Adding a reminder for preventing a misuse of the example in "common gotchas/mutable default arguments"

Author: monchitos

docs/writing/gotchas.rst

@@ -76,6 +76,7 @@ signal that no argument was provided (:py:data:`None` is often a good choice).
         to.append(element)
         return to
 
+Do not forget, you are passing a *list* object as the second argument.
 
 When the Gotcha Isn't a Gotcha
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