Extend guide section on classes regarding free functions

It might not be obvious from the reading the sections on free functions and on
classes that they combine in a frictionless manner, i.e. class instances can be
parameters to free functions in the same manner that the self parameters of
instance methods are handled.

This also explicitly calls out the interaction between `Clone` and
`FromPyObject` for classes.

guide/src/class.md

@@ -227,7 +227,9 @@ struct FrozenCounter {
 }
 
 let py_counter: Py<FrozenCounter> = Python::with_gil(|py| {
-    let counter = FrozenCounter { value: AtomicUsize::new(0) };
+    let counter = FrozenCounter {
+        value: AtomicUsize::new(0),
+    };
 
     Py::new(py, counter).unwrap()
 });
@@ -716,6 +718,67 @@ impl MyClass {
 }
 ```
 
+## Free functions
+
+Free functions defined using `#[pyfunction]` interact with classes through the same mechanisms as the self parameters of instance methods, i.e. they can take GIL-bound references, GIL-bound reference wrappers or GIL-indepedent references:
+
+```rust
+# #![allow(dead_code)]
+# use pyo3::prelude::*;
+#[pyclass]
+struct MyClass {
+    my_field: i32,
+}
+
+// Take a GIL-bound reference when the underlying `PyCell` is irrelevant.
+#[pyfunction]
+fn increment_field(my_class: &mut MyClass) {
+    my_class.my_field += 1;
+}
+
+// Take a GIL-bound reference wrapper when borrowing should be automatic,
+// but interaction with the underlying `PyCell` is desired.
+#[pyfunction]
+fn print_field(my_class: PyRef<'_, MyClass>) {
+    println!("{}", my_class.my_field);
+}
+
+// Take a GIL-bound reference to the underyling cell
+// when borrowing needs to be managed manaually.
+#[pyfunction]
+fn increment_then_print_field(my_class: &PyCell<MyClass>) {
+    my_class.borrow_mut().my_field += 1;
+
+    println!("{}", my_class.borrow().my_field);
+}
+
+// Take a GIL-indepedent reference when you want to store the reference elsewhere.
+#[pyfunction]
+fn print_refcnt(my_class: Py<MyClass>, py: Python<'_>) {
+    println!("{}", my_class.get_refcnt(py));
+}
+```
+
+Classes can also be passed by value if they can be be cloned, i.e. they automatically implement `FromPyObject` if they implement `Clone`, e.g. via `#[derive(Clone)]`:
+
+```rust
+# #![allow(dead_code)]
+# use pyo3::prelude::*;
+#[pyclass]
+#[derive(Clone)]
+struct MyClass {
+    my_field: Box<i32>,
+}
+
+#[pyfunction]
+fn dissamble_clone(my_class: MyClass) {
+    let MyClass { mut my_field } = my_class;
+    *my_field += 1;
+}
+```
+
+Note that `#[derive(FromPyObject)]` on a class is usually not useful as it tries to construct a new Rust value by filling in the fields by looking up attributes of any given Python value.
+
 ## Method arguments
 
 Similar to `#[pyfunction]`, the `#[pyo3(signature = (...))]` attribute can be used to specify the way that `#[pymethods]` accept arguments. Consult the documentation for [`function signatures`](./function/signature.md) to see the parameters this attribute accepts.