[gtkmm/gtkmm-3-24] Button: Explain how to unset image via prop proxy,

[Daniel Boles <dboles src gnome org>]

commit d269d174b8ccac97f244a4fe7da2f34542de650a
Author: Daniel Boles <dboles src gmail com>
Date:   Tue Nov 19 14:41:56 2019 +0000

    Button: Explain how to unset image via prop proxy,
    
    since without being able to add ABI we cannot add unset_image() for that
    
    https://gitlab.gnome.org/GNOME/gtkmm/issues/58
    https://gitlab.gnome.org/GNOME/gtkmm/merge_requests/22#note_650284

 gtk/src/button.hg | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtk/src/button.hg b/gtk/src/button.hg
index c570d666..b9d6eecb 100644
--- a/gtk/src/button.hg
+++ b/gtk/src/button.hg
@@ -144,6 +144,8 @@ public:
    * property_always_show_image() is <tt>true</tt>. You don’t have to call
    * Gtk::Widget::show() on the @a image yourself.
    *
+   * The image can be unset by assigning <tt>nullptr</tt> to property_image().
+   *
    * @newin{2,6}
    *
    * @param image A widget to set as the image for the button.