[`pyflakes`] Add fix safety section (`F841`) (#17410)

add fix safety section to docs for #15584, I'm new to ruff and not sure
if the content of this PR is correct, but I hope it can be helpful.

---------

Co-authored-by: Brent Westbrook <brentrwestbrook@gmail.com>

crates/ruff_linter/src/message/snapshots/ruff_linter__message__sarif__tests__results.snap

@@ -119,7 +119,7 @@ expression: value
             },
             {
               "fullDescription": {
-                "text": "## What it does\nChecks for the presence of unused variables in function scopes.\n\n## Why is this bad?\nA variable that is defined but not used is likely a mistake, and should\nbe removed to avoid confusion.\n\nIf a variable is intentionally defined-but-not-used, it should be\nprefixed with an underscore, or some other value that adheres to the\n[`lint.dummy-variable-rgx`] pattern.\n\n## Example\n```python\ndef foo():\n    x = 1\n    y = 2\n    return x\n```\n\nUse instead:\n```python\ndef foo():\n    x = 1\n    return x\n```\n\n## Options\n- `lint.dummy-variable-rgx`\n"
+                "text": "## What it does\nChecks for the presence of unused variables in function scopes.\n\n## Why is this bad?\nA variable that is defined but not used is likely a mistake, and should\nbe removed to avoid confusion.\n\nIf a variable is intentionally defined-but-not-used, it should be\nprefixed with an underscore, or some other value that adheres to the\n[`lint.dummy-variable-rgx`] pattern.\n\n## Example\n```python\ndef foo():\n    x = 1\n    y = 2\n    return x\n```\n\nUse instead:\n```python\ndef foo():\n    x = 1\n    return x\n```\n\n## Fix safety\n\nThis rule's fix is marked as unsafe because removing an unused variable assignment may\ndelete comments that are attached to the assignment.\n\n## Options\n- `lint.dummy-variable-rgx`\n"
               },
               "help": {
                 "text": "Local variable `{name}` is assigned to but never used"

crates/ruff_linter/src/rules/pyflakes/rules/unused_variable.rs

@@ -38,6 +38,11 @@ use crate::fix::edits::delete_stmt;
 ///     return x
 /// ```
 ///
+/// ## Fix safety
+///
+/// This rule's fix is marked as unsafe because removing an unused variable assignment may
+/// delete comments that are attached to the assignment.
+///
 /// ## Options
 /// - `lint.dummy-variable-rgx`
 #[derive(ViolationMetadata)]