gh-126462: Remove duplicate sentences, fix malformatted example in dataclasses "Post-init processing" section

Doc/library/dataclasses.rst

@@ -504,30 +504,26 @@ 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__()
 
    When defined on the class, it will be called by the generated
-   :meth:`~object.__init__`, normally as ``self.__post_init__()``.
+   :meth:`~object.__init__`, normally as :meth:`!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:`!__init__` method is generated, then
    :meth:`!__post_init__` will not automatically be called.
 
-    @dataclass
-    class C:
+   Among other uses, this allows for initializing field values that
+   depend on one or more other fields.  For example::
 
-        a: float
-        b: float
-        c: float = field(init=False)
+     @dataclass
+     class C:
+         a: float
+         b: float
+         c: float = field(init=False)
 
-        def __post_init__(self):
-            self.c = self.a + self.b
+         def __post_init__(self):
+             self.c = self.a + self.b
 
 The :meth:`~object.__init__` method generated by :func:`@dataclass <dataclass>` does not call base
 class :meth:`!__init__` methods. If the base class has an :meth:`!__init__` method