lint: explain the non-obvious need for a negative test

src/librustc_lint/builtin.rs

@@ -923,6 +923,8 @@ impl NonSnakeCase {
                 allow_underscore = match c {
                     '_' if !allow_underscore => return false,
                     '_' => false,
+                    // It would be more obvious to use `c.is_lowercase()`,
+                    // but some characters do not have a lowercase form
                     c if !c.is_uppercase() => true,
                     _ => return false,
                 };