examples: add some docs to the example using the Properties macro

These will be used later on by the macro.

examples/object_subclass/author.rs

@@ -14,7 +14,12 @@ mod imp {
     #[derive(Properties, Default)]
     #[properties(wrapper_type = super::Author)]
     pub struct Author {
+        /// The name of the author
+        ///
+        /// Just their given name, not their surname.
         #[property(get, set)]
+        /// A helpful name-surname combination.
+        #[property(name = "name-surname", get = |author: &Self| format!("{} {}", author.name.borrow(), author.surname.borrow()))]
         name: RefCell<String>,
         #[property(get, set)]
         surname: RefCell<String>,