[gtk/wip/jimmac/app-icon-helpers] Document icon helper style classes

[Matthias Clasen <matthiasc src gnome org>]

commit c8f91c3f2da0a4bbd9ce64407af5a9c2fa76a066
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Jan 7 20:20:23 2019 -0500

    Document icon helper style classes

 gtk/gtkimage.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkimage.c b/gtk/gtkimage.c
index 2932a0d797..5d6e7150cc 100644
--- a/gtk/gtkimage.c
+++ b/gtk/gtkimage.c
@@ -134,7 +134,8 @@
  *
  * # CSS nodes
  *
- * GtkImage has a single CSS node with the name image.
+ * GtkImage has a single CSS node with the name image. The style classes
+ * may appear on image CSS nodes: .icon-dropshadow, .lowres-icon.
  */