gh-145369: Document that dataclass field objects should not be reused

Add warning to dataclasses documentation explaining that Field objects
returned by field() should not be reused across multiple fields or
classes, as they are modified in place by the decorator.

Author: gourijain029-del

Doc/library/dataclasses.rst

@@ -330,6 +330,27 @@ Module contents
    :attr:`!C.t` will be ``20``, and the class attributes :attr:`!C.x` and
    :attr:`!C.y` will not be set.
 
+   .. warning::
+
+      Do not reuse :class:`Field` objects across multiple fields or classes.
+      Each call to :func:`!field` returns a new object that gets modified
+      in place by the :deco:`dataclass` decorator. Reusing the same field
+      object will cause unexpected behavior::
+
+         f = field(kw_only=True)
+
+         @dataclass
+         class A:
+             x: int = f  # Don't do this
+             y: int = f  # f is now modified and broken
+
+      Instead, call :func:`!field` separately for each field::
+
+         @dataclass
+         class A:
+             x: int = field(kw_only=True)
+             y: int = field(kw_only=True)
+
    .. versionchanged:: next
       If *metadata* is ``None``, use an empty :class:`frozendict`, instead
       of a :func:`~types.MappingProxyType` of an empty :class:`dict`.