[gtk+] Overlay: Document style classes added per position

[Daniel Boles <dboles src gnome org>]

commit 761194dad7e532cabc627c35a38b80af015797b8
Author: Daniel Boles <dboles src gnome org>
Date:   Wed Sep 20 17:34:43 2017 +0100

    Overlay: Document style classes added per position

 gtk/gtkoverlay.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkoverlay.c b/gtk/gtkoverlay.c
index a097a0f..355ad9f 100644
--- a/gtk/gtkoverlay.c
+++ b/gtk/gtkoverlay.c
@@ -53,6 +53,12 @@
  * The GtkOverlay implementation of the GtkBuildable interface
  * supports placing a child as an overlay by specifying “overlay” as
  * the “type” attribute of a `<child>` element.
+ *
+ * # CSS nodes
+ *
+ * GtkOverlay has a single CSS node with the name “overlay”. Overlay children
+ * whose alignments cause them to be positioned at an edge get the style classes
+ * “.left”, “.right”, “.top”, and/or “.bottom” according to their position.
  */
 
 struct _GtkOverlayPrivate