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

[dougthor42]

Addresses this comment:
#11357 (comment)

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:

Test Plan

N/A

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[dougthor42]

For the mkdocs formatting failure:

The multi-line string makes things more obvious IMO. How should I disable the autoformatter for this case? Or is the one-line implicit string concatenation easy enough to read?

[charliermarsh]

We have a way to exclude rules from that check -- I will add it.

[charliermarsh]

Thanks!