Document that accessing `__annotations__` through `__dict__` no longer works

Bug report

Bug description:

The following code crashes on 3.14.0a4 but runs without error on 3.13:

import dataclasses

def my_dataclass(cls):
    try:
        annotations = cls.__dict__["__annotations__"]
    except KeyError:
        cls = dataclasses.dataclass()(cls)
    else:
        new_annotations = dict(annotations)
        for name in annotations.keys():
            if name == "ignore_this_annotation":
                new_annotations.pop(name)
        cls.__annotations__ = new_annotations
        cls = dataclasses.dataclass()(cls)
        cls.__annotations__ = annotations
    return cls

@my_dataclass
class X:
    x: int = 3
    ignore_this_annotation: int

with:

TypeError: non-default argument 'ignore_this_annotation' follows default argument 'x'

Python installed via uv venv --python=3.14.

x-ref https://github.com/patrick-kidger/optimistix/issues/168 https://github.com/patrick-kidger/equinox/issues/1096)

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-151750

先檢視 issue 的重現程式和連結的 PR gh-151750，以了解預期的文件變更。更新相關的 Python 文件，說明透過 dict 存取 annotations 已不再有效，並確認其中正確涵蓋了該行為及版本背景。