[Perflint] Small docs improvement to PERF401 (#18786)

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## Summary

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

While reading the docs I noticed this paragraph on `PERF401`. It was
added in the same PR that the bug with `:=` was fixed, #15050, but don't
know why it was added. The fix should already take care of adding the
parenthesis, so having this paragraph in the docs is just confusing
since it sounds like the user has to do something.

## Test Plan

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

N/A, no tests/functionality affected

crates/ruff_linter/src/rules/perflint/rules/manual_list_comprehension.rs

@@ -48,11 +48,6 @@ use ruff_text_size::{Ranged, TextRange};
 /// original = list(range(10000))
 /// filtered.extend(x for x in original if x % 2)
 /// ```
-///
-/// Take care that if the original for-loop uses an assignment expression
-/// as a conditional, such as `if match:=re.match("\d+","123")`, then
-/// the corresponding comprehension must wrap the assignment
-/// expression in parentheses to avoid a syntax error.
 #[derive(ViolationMetadata)]
 pub(crate) struct ManualListComprehension {
     is_async: bool,