faq: Clarify how Ruff and Black treat line-length.

As discussed at micropython/micropython#10977 (comment)

docs/faq.md

@@ -3,7 +3,9 @@
 ## Is Ruff compatible with Black?
 
 Yes. Ruff is compatible with [Black](https://github.com/psf/black) out-of-the-box, as long as
-the `line-length` setting is consistent between the two.
+the `line-length` setting is consistent between the two.  Ruff and Black treat `line-length`
+differently because Black does not wrap all long lines (for example, comments) while Ruff will
+still flag those as long lines.
 
 As a project, Ruff is designed to be used alongside Black and, as such, will defer implementing
 stylistic lint rules that are obviated by autoformatting.