gh-114071: [Enum] update docs and code for tuples/subclasses (GH-114871)

Update documentation with `__new__` and `__init__` entries.

Support use of `auto()` in tuple subclasses on member assignment lines.  Previously, auto() was only supported on the member definition line either solo or as part of a tuple:

    RED = auto()
    BLUE = auto(), 'azul'

However, since Python itself supports using tuple subclasses where tuples are expected, e.g.:

    from collections import namedtuple
    T = namedtuple('T', 'first second third')

    def test(one, two, three):
        print(one, two, three)

    test(*T(4, 5, 6))
    # 4 5 6

it made sense to also support tuple subclasses in enum definitions.