examples: Add a few more docs/comments to the subclass/virtual methods example

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1366>

examples/src/bin/subclass_vfuncs/iirfilter/imp.rs

@@ -45,6 +45,8 @@ impl ObjectSubclass for IirFilter {
     type ParentType = gst_audio::AudioFilter;
     type Class = super::Class;
 
+    // Here we set default implementations for all the virtual methods.
+    // This is mandatory for all virtual methods that are not `Option`s.
     fn class_init(class: &mut Self::Class) {
         class.set_rate = |obj, rate| obj.imp().set_rate_default(rate);
     }

examples/src/bin/subclass_vfuncs/iirfilter/mod.rs

@@ -60,6 +60,8 @@ unsafe impl ClassStruct for Class {
     type Type = imp::IirFilter;
 }
 
+/// This allows directly using `Class` as e.g. `gst_audio::AudioFilterClass` or
+/// `gst_base::BaseTransformClass` without having to cast.
 impl std::ops::Deref for Class {
     type Target = glib::Class<<<Self as ClassStruct>::Type as ObjectSubclass>::ParentType>;
 
@@ -68,6 +70,8 @@ impl std::ops::Deref for Class {
     }
 }
 
+/// Overrides the virtual methods with the actual implementation of the subclass as is provided by
+/// the subclass' implementation of the `Impl` trait.
 unsafe impl<T: IirFilterImpl> IsSubclassable<T> for IirFilter {
     fn class_init(class: &mut glib::Class<Self>) {
         Self::parent_class_init::<T>(class);

examples/src/bin/subclass_vfuncs/lowpass/imp.rs

@@ -121,6 +121,11 @@ impl AudioFilterImpl for Lowpass {}
 // Implement of IirFilter virtual methods
 impl IirFilterImpl for Lowpass {
     fn set_rate(&self, rate: u32) {
+        // Could call
+        //   self.parent_set_rate(rate);
+        // here but chaining up is not necessary if the base class doesn't require that
+        // or if the behaviour of the parent class should be completely overridden.
+
         self.state.lock().unwrap().rate = Some(rate);
         self.calculate_coeffs();
     }