[3.7] bpo-37814: Document the empty tuple type annotation syntax (GH-15208) (GH-15262)

[3.7] bpo-37814: Document the empty tuple type annotation syntax (GH-15208) (GH-15262)

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
(cherry picked from commit 8a784af750fa82c8355903309e5089eb2b60c16b)

Co-authored-by: Josh Holland <anowlcalledjosh@gmail.com>
https://bugs.python.org/issue37814

Automerge-Triggered-By: @gvanrossum