add doc comment detail

Author: estebank

Diff for: compiler/rustc_errors/src/diagnostic.rs

@@ -147,6 +147,11 @@ where
 /// Implemented as a custom trait rather than `From` so that it is implemented on the type being
 /// converted rather than on `DiagArgValue`, which enables types from other `rustc_*` crates to
 /// implement this.
+///
+/// It takes a `path` where "long values" could have been written to, if the `DiagArgValue` would be
+/// too big for displaying on the terminal. This path comes from the `Diag` itself. When rendering
+/// values that come from `TyCtxt`, like `Ty<'_>`, they can use `TyCtxt::short_string`. If a value
+/// has no shortening logic that could be used, the argument can be safely ignored.
 pub trait IntoDiagArg {
     fn into_diag_arg(self, path: &mut Option<std::path::PathBuf>) -> DiagArgValue;
 }