[RUF008] Make it clearer that a mutable default in a dataclass is onl…

…y valid if it is typed as a ClassVar (#10395)

## Summary

The previous documentation sounded as if typing a mutable default as a
`ClassVar` were optional. However, it is not, as not doing so causes a
`ValueError`. The snippet below was tested in Python's interactive
shell:

```
>>> from dataclasses import dataclass
>>> @DataClass
... class A:
...     mutable_default: list[int] = []
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/dataclasses.py", line 1230, in dataclass
    return wrap(cls)
           ^^^^^^^^^
  File "/usr/lib/python3.11/dataclasses.py", line 1220, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/dataclasses.py", line 958, in _process_class
    cls_fields.append(_get_field(cls, name, type, kw_only))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/dataclasses.py", line 815, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class 'list'> for field mutable_default is not allowed: use default_factory
>>>
```

This behavior is also documented in Python's docs, see
[here](https://docs.python.org/3/library/dataclasses.html#mutable-default-values):

> [...] the
[dataclass()](https://docs.python.org/3/library/dataclasses.html#dataclasses.dataclass)
decorator will raise a
[ValueError](https://docs.python.org/3/library/exceptions.html#ValueError)
if it detects an unhashable default parameter. The assumption is that if
a value is unhashable, it is mutable. This is a partial solution, but it
does protect against many common errors.

And
[here](https://docs.python.org/3/library/dataclasses.html#class-variables)
it is documented why it works if it is typed as a `ClassVar`:

> One of the few places where
[dataclass()](https://docs.python.org/3/library/dataclasses.html#dataclasses.dataclass)
actually inspects the type of a field is to determine if a field is a
class variable as defined in [PEP
526](https://peps.python.org/pep-0526/). It does this by checking if the
type of the field is typing.ClassVar. If a field is a ClassVar, it is
excluded from consideration as a field and is ignored by the dataclass
mechanisms. Such ClassVar pseudo-fields are not returned by the
module-level
[fields()](https://docs.python.org/3/library/dataclasses.html#dataclasses.fields)
function.

In this PR I have changed the documentation to make it a little bit
clearer that not using `ClassVar` makes the code invalid.

crates/ruff_linter/src/rules/ruff/rules/mutable_dataclass_default.rs

@@ -19,8 +19,8 @@ use crate::rules::ruff::rules::helpers::{is_class_var_annotation, is_dataclass};
 /// Instead of sharing mutable defaults, use the `field(default_factory=...)`
 /// pattern.
 ///
-/// If the default value is intended to be mutable, it should be annotated with
-/// `typing.ClassVar`.
+/// If the default value is intended to be mutable, it must be annotated with
+/// `typing.ClassVar`; otherwise, a `ValueError` will be raised.
 ///
 /// ## Examples
 /// ```python
@@ -29,6 +29,8 @@ use crate::rules::ruff::rules::helpers::{is_class_var_annotation, is_dataclass};
 ///
 /// @dataclass
 /// class A:
+/// # A list without a `default_factory` or `ClassVar` annotation
+/// # will raise a `ValueError`.
 /// mutable_default: list[int] = []
 /// ```
 ///
@@ -44,7 +46,7 @@ use crate::rules::ruff::rules::helpers::{is_class_var_annotation, is_dataclass};
 ///
 /// Or:
 /// ```python
-/// from dataclasses import dataclass, field
+/// from dataclasses import dataclass
 /// from typing import ClassVar
 ///
 ///