[pycodestyle] Add fix safety section to W291 and W293 (#18800)

Part of #15584

This PR adds fix safety sections to `W291` and `W293`

The unsafe caveat was added in #10049


10a1d9f01e/crates/ruff_linter/src/rules/pycodestyle/rules/trailing_whitespace.rs (L92)

Code example demonstrating unsafety:
```
PS ~\Desktop\New_folder\ruff>Get-Content issue.py
```
```py
# W291
"""
1
"""

# W293
"""

"""
```
```
PS ~\Desktop\New_folder\ruff>Get-Escaped-Content issue.py
```
```
# W291\n"""\n1 \n"""\n\n# W293\n"""\n \n"""\r\n
```
```
PS ~\Desktop\New_folder\ruff>uvx ruff check issue.py --isolated --select W
```
```snap
issue.py:3:2: W291 Trailing whitespace
  |
1 | # W291
2 | """
3 | 1
  |  ^ W291
4 | """
  |
  = help: Remove trailing whitespace

issue.py:8:1: W293 Blank line contains whitespace
  |
6 | # W293
7 | """
8 |
  | ^ W293
9 | """
  |
  = help: Remove whitespace from blank line

Found 2 errors.
No fixes available (2 hidden fixes can be enabled with the `--unsafe-fixes` option).
```

## Test Plan

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

N/A, no tests affected.

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

@@ -26,6 +26,11 @@ use crate::{AlwaysFixableViolation, Applicability, Edit, Fix};
 /// spam(1)\n#
 /// ```
 ///
+/// ## Fix Safety
+///
+/// This fix is marked unsafe if the whitespace is inside a multiline string,
+/// as removing it changes the string's content.
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/#other-recommendations
 #[derive(ViolationMetadata)]
 pub(crate) struct TrailingWhitespace;
@@ -58,6 +63,11 @@ impl AlwaysFixableViolation for TrailingWhitespace {
 /// class Foo(object):\n\n    bang = 12
 /// ```
 ///
+/// ## Fix Safety
+///
+/// This fix is marked unsafe if the whitespace is inside a multiline string,
+/// as removing it changes the string's content.
+///
 /// [PEP 8]: https://peps.python.org/pep-0008/#other-recommendations
 #[derive(ViolationMetadata)]
 pub(crate) struct BlankLineWithWhitespace;