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gh-132111: Document dataclasses.InitVar (#132446)

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Doc/library/dataclasses.rst
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@ -344,6 +344,15 @@ Module contents
Other attributes may exist, but they are private and must not be
inspected or relied on.
.. class:: InitVar
``InitVar[T]`` type annotations describe variables that are :ref:`init-only
<dataclasses-init-only-variables>`. Fields annotated with :class:`!InitVar`
are considered pseudo-fields, and thus are neither returned by the
:func:`fields` function nor used in any way except adding them as
parameters to :meth:`~object.__init__` and an optional
:meth:`__post_init__`.
.. function:: fields(class_or_instance)
Returns a tuple of :class:`Field` objects that define the fields for this
@ -600,8 +609,8 @@ Init-only variables
Another place where :func:`@dataclass <dataclass>` inspects a type annotation is to
determine if a field is an init-only variable. It does this by seeing
if the type of a field is of type ``dataclasses.InitVar``. If a field
is an ``InitVar``, it is considered a pseudo-field called an init-only
if the type of a field is of type :class:`InitVar`. If a field
is an :class:`InitVar`, it is considered a pseudo-field called an init-only
field. As it is not a true field, it is not returned by the
module-level :func:`fields` function. Init-only fields are added as
parameters to the generated :meth:`~object.__init__` method, and are passed to