gh-101100: Add reference doc for __post_init__ (#103818)

Signed-off-by: Olga Matoula <olgamatoula@gmail.com>

Doc/library/dataclasses.rst

@@ -437,11 +437,11 @@ Module contents
 
    The newly returned object is created by calling the :meth:`~object.__init__`
    method of the dataclass.  This ensures that
-   :ref:`__post_init__ <post-init-processing>`, if present, is also called.
+   :meth:`__post_init__`, if present, is also called.
 
    Init-only variables without default values, if any exist, must be
    specified on the call to :func:`replace` so that they can be passed to
-   :meth:`~object.__init__` and :ref:`__post_init__ <post-init-processing>`.
+   :meth:`~object.__init__` and :meth:`__post_init__`.
 
    It is an error for ``changes`` to contain any fields that are
    defined as having ``init=False``.  A :exc:`ValueError` will be raised
@@ -449,7 +449,7 @@ Module contents
 
    Be forewarned about how ``init=False`` fields work during a call to
    :func:`replace`.  They are not copied from the source object, but
-   rather are initialized in :ref:`__post_init__ <post-init-processing>`, if they're
+   rather are initialized in :meth:`__post_init__`, if they're
    initialized at all.  It is expected that ``init=False`` fields will
    be rarely and judiciously used.  If they are used, it might be wise
    to have alternate class constructors, or perhaps a custom
@@ -510,30 +510,31 @@ Module contents
 Post-init processing
 --------------------
 
-The generated :meth:`~object.__init__` code will call a method named
-:meth:`!__post_init__`, if :meth:`!__post_init__` is defined on the
-class.  It will normally be called as ``self.__post_init__()``.
-However, if any ``InitVar`` fields are defined, they will also be
-passed to :meth:`!__post_init__` in the order they were defined in the
-class.  If no :meth:`~object.__init__` method is generated, then
-:meth:`!__post_init__` will not automatically be called.
+.. function:: __post_init__()
 
-Among other uses, this allows for initializing field values that
-depend on one or more other fields.  For example::
+   When defined on the class, it will be called by the generated
+   :meth:`~object.__init__`, normally as ``self.__post_init__()``.
+   However, if any ``InitVar`` fields are defined, they will also be
+   passed to :meth:`__post_init__` in the order they were defined in the
+   class.  If no :meth:`~object.__init__` method is generated, then
+   :meth:`__post_init__` will not automatically be called.
 
-    @dataclass
-    class C:
-        a: float
-        b: float
-        c: float = field(init=False)
+   Among other uses, this allows for initializing field values that
+   depend on one or more other fields.  For example::
 
-        def __post_init__(self):
-            self.c = self.a + self.b
+     @dataclass
+     class C:
+         a: float
+         b: float
+         c: float = field(init=False)
+
+         def __post_init__(self):
+             self.c = self.a + self.b
 
 The :meth:`~object.__init__` method generated by :func:`dataclass` does not call base
 class :meth:`~object.__init__` methods. If the base class has an :meth:`~object.__init__` method
 that has to be called, it is common to call this method in a
-:meth:`!__post_init__` method::
+:meth:`__post_init__` method::
 
     @dataclass
     class Rectangle:
@@ -552,7 +553,7 @@ don't need to be called, since the derived dataclass will take care of
 initializing all fields of any base class that is a dataclass itself.
 
 See the section below on init-only variables for ways to pass
-parameters to :meth:`!__post_init__`.  Also see the warning about how
+parameters to :meth:`__post_init__`.  Also see the warning about how
 :func:`replace` handles ``init=False`` fields.
 
 Class variables
@@ -576,7 +577,7 @@ is an ``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
-the optional :ref:`__post_init__ <post-init-processing>` method.  They are not otherwise used
+the optional :meth:`__post_init__` method.  They are not otherwise used
 by dataclasses.
 
 For example, suppose a field will be initialized from a database, if a