[gtk+] Document the .circular style class

[Matthias Clasen <matthiasc src gnome org>]

commit 0b156ea0962066943b21d16818409eb3afe3c71c
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Mar 2 19:10:59 2016 -0500

    Document the .circular style class

 gtk/gtkbutton.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtkbutton.c b/gtk/gtkbutton.c
index 56452bb..f552e7b 100644
--- a/gtk/gtkbutton.c
+++ b/gtk/gtkbutton.c
@@ -42,7 +42,8 @@
  * image or label, respectively. It may also receive the .flat style class.
  *
  * Other style classes that are commonly used with GtkButton include
- * .suggested-action and .destructive-action.
+ * .suggested-action and .destructive-action. In special cases, buttons
+ * can be made round by adding the .circular style class.
  *
  * Button-like widgets like #GtkToggleButton, #GtkMenuButton, #GtkVolumeButton,
  * #GtkLockButton, #GtkColorButton, #GtkFontButton or #GtkFileChooserButton use