[gtk+] Spinner: Document how to select on animated in CSS

[Daniel Boles <dboles src gnome org>]

commit d48f904e3c39fc7074601ca2e876296a76d55ec2
Author: Daniel Boles <dboles src gnome org>
Date:   Thu Sep 14 19:22:31 2017 +0100

    Spinner: Document how to select on animated in CSS
    
    This is not particularly obvious, so it seems worth including.

 gtk/gtkspinner.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtkspinner.c b/gtk/gtkspinner.c
index 3b4fc7d..24a4920 100644
--- a/gtk/gtkspinner.c
+++ b/gtk/gtkspinner.c
@@ -57,7 +57,8 @@
  *
  * # CSS nodes
  *
- * GtkSpinner has a single CSS node with the name spinner.
+ * GtkSpinner has a single CSS node with the name spinner. When the animation is
+ * active, the :checked pseudoclass is added to this node.
  */