Add more detailed explanations for #13885 regression test

Author: GuillaumeGomez

tests/ui/{literal_string_with_formatting_args}.rs

@@ -2,6 +2,38 @@
 // The `dbg` macro generates a literal with the name of the current file, so
 // we need to ensure the lint is not emitted in this case.
 
+// Clippy sets `-Zflatten_format_args=no`, which changes the default behavior of how format args
+// are lowered and only that one has this non-macro span. Adding the flag makes it repro on
+// godbolt and shows a root context span for the file name string.
+//
+// So instead of having:
+//
+// ```
+// Lit(
+//     Spanned {
+//         node: Str(
+//             "[/app/example.rs:2:5] \"something\" = ",
+//             Cooked,
+//         ),
+//         span: /rustc/eb54a50837ad4bcc9842924f27e7287ca66e294c/library/std/src/macros.rs:365:35: 365:58 (#4),
+//     },
+// ),
+// ```
+//
+// We get:
+//
+// ```
+// Lit(
+//     Spanned {
+//         node: Str(
+//             "/app/example.rs",
+//             Cooked,
+//         ),
+//         span: /app/example.rs:2:5: 2:22 (#0),
+//     },
+// )
+// ```
+
 #![crate_name = "foo"]
 #![allow(unused)]
 #![warn(clippy::literal_string_with_formatting_args)]