[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](https://github.com/astral-sh/ruff/assets/5386897/32067121-b190-4268-b987-ff37df11a618)

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",