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.

Doc/library/enum.rst

@@ -337,6 +337,17 @@ Data Types
          >>> PowersOfThree.SECOND.value
          9
 
+   .. method:: Enum.__init__(self, \*args, \**kwds)
+
+      By default, does nothing.  If multiple values are given in the member
+      assignment, those values become separate arguments to ``__init__``; e.g.
+
+         >>> from enum import Enum
+         >>> class Weekday(Enum):
+         ...     MONDAY = 1, 'Mon'
+
+      ``Weekday.__init__()`` would be called as ``Weekday.__init__(self, 1, 'Mon')``
+
    .. method:: Enum.__init_subclass__(cls, \**kwds)
 
       A *classmethod* that is used to further configure subsequent subclasses.
@@ -364,6 +375,18 @@ Data Types
          >>> Build('deBUG')
          <Build.DEBUG: 'debug'>
 
+   .. method:: Enum.__new__(cls, \*args, \**kwds)
+
+      By default, doesn't exist.  If specified, either in the enum class
+      definition or in a mixin class (such as ``int``), all values given
+      in the member assignment will be passed; e.g.
+
+         >>> from enum import Enum
+         >>> class MyIntEnum(Enum):
+         ...     SEVENTEEN = '1a', 16
+
+      results in the call ``int('1a', 16)`` and a value of ``17`` for the member.
+
    .. method:: Enum.__repr__(self)
 
       Returns the string used for *repr()* calls.  By default, returns the
@@ -477,9 +500,9 @@ Data Types
 
 .. class:: Flag
 
-   *Flag* members support the bitwise operators ``&`` (*AND*), ``|`` (*OR*),
-   ``^`` (*XOR*), and ``~`` (*INVERT*); the results of those operators are members
-   of the enumeration.
+   ``Flag`` is the same as :class:`Enum`, but its members support the bitwise
+   operators ``&`` (*AND*), ``|`` (*OR*), ``^`` (*XOR*), and ``~`` (*INVERT*);
+   the results of those operators are members of the enumeration.
 
    .. method:: __contains__(self, value)