Clairfy when `DictComp.expr` value is `None` in the `ast` docs

[mushitoriami]

Documentation

In Python 3.15 (via PEP 798), the value parameter type of ast.DictComp was changed from expr to expr | None to support unpacking in dict comprehensions. However, the current ast module documentation does not mention this change. It would be helpful to show what kind of Python code causes value to be None.

[picnixz]

Actually, the grammar is consistent. It says:

     | DictComp(expr key, expr? value, comprehension* generators)

And we don't put the optional fields in the ast def otherwise, or am I wrong?

[picnixz]

For instance, the orelse branch in https://docs.python.org/3.16/library/ast.html#ast.If is not spelled as None. However, I agree we could spell it explicitly that value is None when unpacking in comprehensions.