bpo-37814: Document the empty tuple type annotation syntax (GH-15208)

https://bugs.python.org/issue37814:

> The empty tuple syntax in type annotations, `Tuple[()]`, is not obvious from the examples given in the documentation (I naively expected `Tuple[]` to work); it has been documented in PEP 484 and in mypy, but not in the documentation for the typing module.


https://bugs.python.org/issue37814