[flake8-pie] Add fix safety section to PIE794 (astral-sh#18802)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Part of astral-sh#15584

This PR adds a fix safety section to `PIE794`

I could not track down when this rule was initially implemented/made
unsafe due how old it could be + multiple large refactors to `ruff`.

There is no comment/reasoning in the code given for the unsafety.

Here is a code example demonstrating why it should be unsafe, since
removing any of the assignments would change program behavior
[playground](https://play.ruff.rs/01004644-4259-4449-a581-5007cd59846a)
```py
class A:
    x = 1
    x = 2
    print(x)

class B:
    x = print(3)
    x = print(4)

class C:
    x = [1,2,3]
    y = x
    x = y[1]
```

## Test Plan

<!-- How was it tested? -->

N/A, no tests affected.

---------

Co-authored-by: Dylan <[email protected]>

Author: MeGaGiGaGon

crates/ruff_linter/src/rules/flake8_pie/rules/duplicate_class_field_definition.rs

@@ -30,6 +30,10 @@ use crate::{AlwaysFixableViolation, Fix};
 ///     name = Tom
 ///     ...
 /// ```
+///
+/// ## Fix Safety
+/// This fix is always marked as unsafe since we cannot know
+//  for certain which assignment was intended.
 #[derive(ViolationMetadata)]
 pub(crate) struct DuplicateClassFieldDefinition {
     name: String,