Add section about not including sources in Display impl

Author: faern

src/interoperability.md

@@ -273,7 +273,27 @@ The message given by [`Error::description()`] does not matter. Users should
 always use `Display` instead of `description()` to print the error. A low-effort
 description like `"JSON error"` is sufficient.
 
+If an error type returns an underlying source error from [`Error::source()`], it does **not**
+include that source error in its own `Display` representation as well. This avoids duplicated
+information when an error is printed along with all its sources:
+
+```rust
+use std::fmt;
+
+impl fmt::Display for ParseError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Good errors never duplicate information like this:
+        write!(f, "failed to parse: {}", self.source),
+
+        // Instead they just describe themselves:
+        write!(f, "failed to parse"),
+    }
+}
+```
+
 [`Error::description()`]: https://doc.rust-lang.org/std/error/trait.Error.html#tymethod.description
+[`Error::source()`]: https://doc.rust-lang.org/std/error/trait.Error.html#method.source
+
 
 ### Examples from the standard library