docs: Clarify child objects in GtkBuilder

Also, document the GListStore special case.

gtk/gtkbuilder.c

@@ -99,9 +99,7 @@
  * Objects are described by `<object>` elements, which can contain
  * `<property>` elements to set properties, `<signal>` elements which
  * connect signals to handlers, and `<child>` elements, which describe
- * child objects (most often widgets inside a container, but also e.g.
- * actions in an action group, or columns in a tree model). A `<child>`
- * element contains an `<object>` element which describes the child object.
+ * child objects.
  *
  * Typically, the specific kind of object represented by an `<object>`
  * element is specified by the “class” attribute. If the type has not
@@ -173,6 +171,38 @@
  * exception to this rule is that an object has to be constructed before
  * it can be used as the value of a construct-only property.
  *
+ * ### Child objects
+ *
+ * A `<child>` element contains an `<object>` element which describes the
+ * child object. Most often, child objects are widgets inside a container,
+ * but they can also be, e.g., actions in an action group, or columns in a
+ * tree model.
+ *
+ * Any object type that implements the [iface@Gtk.Buildable] interface can
+ * specify how children may be added to it. Since many objects and widgets that
+ * are included with GTK already implement the `GtkBuildable` interface,
+ * typically child objects can be added using the `<child>` element without
+ * having to be concerned about the underlying implementation.
+ *
+ * See the [`GtkWidget` documentation](class.Widget.html#gtkwidget-as-gtkbuildable)
+ * for many examples of using `GtkBuilder` with widgets, including setting
+ * child objects using the `<child>` element.
+ *
+ * A noteworthy special case to the general rule that only objects implementing
+ * `GtkBuildable` may specify how to handle the `<child>` element is that
+ * `GtkBuilder` provides special support for adding objects to a
+ * [class@Gio.ListStore] by using the `<child>` element. For instance:
+ *
+ * ```xml
+ * <object class="GListStore">
+ *   <property name="item-type">MyObject</property>
+ *   <child>
+ *     <object class="MyObject" />
+ *   </child>
+ *   ...
+ * </object>
+ * ```
+ *
  * ### Property bindings
  *
  * It is also possible to bind a property value to another object's