docs: Mention anyhow for error handling in UT doc
Add a comment stating that `anyhow` and `thiserror` should be used in
real rust code, rather than the unwieldy default `Result` handling
shown in the example.
@@ -120,6 +120,8 @@ Assume the following function:
```rust
// Convenience type to allow Result return types to only specify the type
// for the true case; failures are specified as static strings.
+// XXX: This is an example. In real code use the "anyhow" and
+// XXX: "thiserror" crates.
pub type Result<T> = std::result::Result<T, &'static str>;
// The function under test.
