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

Start by reviewing the issue's reproducer and linked PR gh-151750 to understand the intended documentation change. Update the relevant Python documentation to explain that accessing annotations through dict no longer works, and confirm the behavior and version context are accurately covered.