docs: demonstrate `NotImplemented` in `__richcmp__`

guide/src/class/object.md

@@ -174,6 +174,30 @@ impl Number {
 It checks that the `std::cmp::Ordering` obtained from Rust's `Ord` matches
 the given `CompareOp`.
 
+Alternatively, if you want to leave some operations unimplemented, you can
+return `py.NotImplemented()` for some of the operations:
+
+
+```rust
+use pyo3::class::basic::CompareOp;
+
+# use pyo3::prelude::*;
+#
+# #[pyclass]
+# struct Number(i32);
+#
+#[pymethods]
+impl Number {
+    fn __richcmp__(&self, other: &Self, op: CompareOp, py: Python<'_>) -> PyObject {
+        match op {
+            CompareOp::Eq => (self.0 == other.0).into_py(py),
+            CompareOp::Ne => (self.0 != other.0).into_py(py),
+            _ => py.NotImplemented(),
+        }
+    }
+}
+```
+
 ### Truthyness
 
 We'll consider `Number` to be `True` if it is nonzero: