add doc comment detail

Author: estebank

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

@@ -148,6 +148,12 @@ where
 /// converted rather than on `DiagArgValue`, which enables types from other `rustc_*` crates to
 /// implement this.
 pub trait IntoDiagArg {
+    /// Convert `Self` into a `DiagArgValue` suitable for rendering in a diagnostic.
+    ///
+    /// It takes a `path` where "long values" could be written to, if the `DiagArgValue` is 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.
     fn into_diag_arg(self, path: &mut Option<std::path::PathBuf>) -> DiagArgValue;
 }