[pyflakes] Update docs to describe WAI behavior (F541) (#11362)

Addresses this comment:
https://github.com/astral-sh/ruff/issues/11357#issuecomment-2104714029


## Summary

The docs for F541 did not mention some surprising, but WAI, behavior
regarding implicit string concatenation. Update the docs to describe the
behavior.

Here's how things rendered for me locally:


![image](32067121-b190-4268-b987-ff37df11a618)

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

@@ -29,6 +29,28 @@ use crate::checkers::ast::Checker;
 /// "Hello, world!"
 /// ```
 ///
+/// **Note:** to maintain compatibility with PyFlakes, this rule only flags
+/// f-strings that are part of an implicit concatenation if _none_ of the
+/// f-string segments contain placeholder expressions.
+///
+/// For example:
+///
+/// ```python
+/// # Will not be flagged.
+/// (
+///     f"Hello,"
+///     f" {name}!"
+/// )
+///
+/// # Will be flagged.
+/// (
+///     f"Hello,"
+///     f" World!"
+/// )
+/// ```
+///
+/// See [#10885](https://github.com/astral-sh/ruff/issues/10885) for more.
+///
 /// ## References
 /// - [PEP 498](https://www.python.org/dev/peps/pep-0498/)
 #[violation]

scripts/check_docs_formatted.py

@@ -39,6 +39,7 @@ KNOWN_FORMATTING_VIOLATIONS = [
     "blank-lines-before-nested-definition",
     "blank-lines-top-level",
     "explicit-string-concatenation",
+    "f-string-missing-placeholders",
     "indent-with-spaces",
     "indentation-with-invalid-multiple",
     "line-too-long",