Ivan, 

I believe, this sentence : "(the scope in which a new binding should be created cannot be determined unambiguously)" in https://docs.python.org/fr/3/reference/simple_stmts.html#nonlocal, is the one which should be clarified first, and then this would probably give the bases to improve the error message.
It raises the following concerns : 
  1/ Is it me or there is a nasty double negation in this sentence which make it tell the contrary of what it should ?
  2/ What are the conditions which would make such an ambiguity appear ? I wonder what the persons who wrote this had on their mind when they wrote it, but in regard with code reading/reviewing, my original example might be an example of such an ambiguous situation. Now the question is: are there others and what would they look like ?