Update docstrings of typing.Hashable, typing.List, etc., to reflect their deprecation

[AlexWaygood]

Currently some of the typing-module stdlib aliases have docstrings that reflect their deprecation, but others inherit a generic docstring that does not mention any deprecation (and implies that they add something over and above the stdlib class that they're aliasing):

>>> from typing import *
>>> help(Tuple)
Help on _TupleType in module typing:

Tuple = typing.Tuple
    Deprecated alias to builtins.tuple.

    Tuple[X, Y] is the cross-product type of X and Y.

    Example: Tuple[T1, T2] is a tuple of two elements corresponding
    to type variables T1 and T2.  Tuple[int, float, str] is a tuple
    of an int, a float and a string.

    To specify a variable-length tuple of homogeneous type, use Tuple[T, ...].

>>> help(Hashable)
Help on _SpecialGenericAlias in module typing:

Hashable = typing.Hashable
    A generic version of collections.abc.Hashable.

This PR improves the generic fallback docstring so that it's consistent with the "manual" docstrings we give for the aliases like Tuple and Callable:

>>> from typing import *
>>> help(Tuple)
Help on _TupleType in module typing:

Tuple = typing.Tuple
    Deprecated alias to builtins.tuple.

    Tuple[X, Y] is the cross-product type of X and Y.

    Example: Tuple[T1, T2] is a tuple of two elements corresponding
    to type variables T1 and T2.  Tuple[int, float, str] is a tuple
    of an int, a float and a string.

    To specify a variable-length tuple of homogeneous type, use Tuple[T, ...].

>>> help(Hashable)
Help on _SpecialGenericAlias in module typing:

Hashable = typing.Hashable
    Deprecated alias to collections.abc.Hashable.