[gtk+/gtk-3-22] Spinner: Document how to select on animated in CSS

[Daniel Boles <dboles src gnome org>]

commit f2a4b2f14cb2178266327d2196e3b4c086a6983f
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 0f410e0..06020b3 100644
--- a/gtk/gtkspinner.c
+++ b/gtk/gtkspinner.c
@@ -56,7 +56,8 @@
  *
  * # 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.
  */