[gtkmm] Builder: Document get_widget_derived() builder arg

[Daniel Boles <dboles src gnome org>]

commit a8bb760c3311d4badbd87abef3276f5fa45bc5f5
Author: Daniel Boles <dboles src gnome org>
Date:   Mon Dec 17 01:45:50 2018 +0000

    Builder: Document get_widget_derived() builder arg
    
    This function being static is basically a workaround to avoid inheriting
    std::make_shared_from_this. We didn't add the @param builder during that

 gtk/src/builder.hg | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/src/builder.hg b/gtk/src/builder.hg
index 1ca53b5f..0f81e701 100644
--- a/gtk/src/builder.hg
+++ b/gtk/src/builder.hg
@@ -510,6 +510,7 @@ public:
    * The following calls will return a pointer to the widget instance that
    * was constructed by the first call.
    *
+   * @param builder The Gtk::Builder from which to get the widget.
    * @param name The name of the widget.
    * @param args Additional arguments to pass to the constructor of the derived class.
    * @retval widget A pointer to the widget, or <tt>nullptr</tt> on failure.