[gtk+] Expander: Explain how to conditionally style arrow

[Daniel Boles <dboles src gnome org>]

commit 3cd117e3a153dda019fec0a894d3bc0dec2059ac
Author: Daniel Boles <dboles src gnome org>
Date:   Thu Sep 14 21:11:50 2017 +0100

    Expander: Explain how to conditionally style arrow

 gtk/gtkexpander.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkexpander.c b/gtk/gtkexpander.c
index e1f3617..bf29555 100644
--- a/gtk/gtkexpander.c
+++ b/gtk/gtkexpander.c
@@ -99,8 +99,8 @@
  * ]|
  *
  * GtkExpander has three CSS nodes, the main node with the name expander,
- * a subnode with name title and node below it with name arrow. Neither of
- * them is using any style classes.
+ * a subnode with name title and node below it with name arrow. The arrow of an
+ * expander that is showing its child gets the :checked pseudoclass added to it.
  */
 
 #include "config.h"