Update docs for E711, E712 (astral-sh#4560)

The fixes for rules E711 `NoneComparison` and E712 `TrueFalseComparison` are marked unsafe due to possible runtime behavior changes with libraries that override `__eq__` and `__ne__` methods.

- Add a "Fix safety" section to each rule explaining why the fixes are unsafe, plus a link to a GH issue with more detail. The sections are identical for each rule.
- Minor formatting tweak for E711's docs.

crates/ruff_linter/src/rules/pycodestyle/rules/literal_comparisons.rs

@@ -32,7 +32,7 @@ impl EqCmpOp {
 ///
 /// ## Why is this bad?
 /// According to [PEP 8], "Comparisons to singletons like None should always be done with
-/// is or is not, never the equality operators."
+/// `is` or `is not`, never the equality operators."
 ///
 /// ## Example
 /// ```python
@@ -48,7 +48,15 @@ impl EqCmpOp {
 ///     pass
 /// ```
 ///
+/// ## Fix safety
+///
+/// This rule's fix is marked as unsafe, as it may alter runtime behavior when
+/// used with libraries that override the `==`/`__eq__` or `!=`/`__ne__` operators.
+/// In these cases, `is`/`is not` may not be equivalent to `==`/`!=`. For more
+/// information, see [this issue].
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/#programming-recommendations
+/// [this issue]: https://github.com/astral-sh/ruff/issues/4560
 #[violation]
 pub struct NoneComparison(EqCmpOp);
 
@@ -101,7 +109,15 @@ impl AlwaysFixableViolation for NoneComparison {
 ///     ...
 /// ```
 ///
+/// ## Fix safety
+///
+/// This rule's fix is marked as unsafe, as it may alter runtime behavior when
+/// used with libraries that override the `==`/`__eq__` or `!=`/`__ne__` operators.
+/// In these cases, `is`/`is not` may not be equivalent to `==`/`!=`. For more
+/// information, see [this issue].
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/#programming-recommendations
+/// [this issue]: https://github.com/astral-sh/ruff/issues/4560
 #[violation]
 pub struct TrueFalseComparison {
     value: bool,