[gtk+] Frame: minor update to intro docs

[Daniel Boles <dboles src gnome org>]

commit 24b966f6e7e7819bbd31c0205baf6eef9e8effe1
Author: Daniel Boles <dboles src gnome org>
Date:   Mon Mar 6 07:24:17 2017 +0000

    Frame: minor update to intro docs
    
    This incorporates a couple of recent tweaks from gtk-3-22.

 gtk/gtkframe.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkframe.c b/gtk/gtkframe.c
index 8489730..9c7ec93 100644
--- a/gtk/gtkframe.c
+++ b/gtk/gtkframe.c
@@ -79,8 +79,9 @@
  * ]|
  *
  * GtkFrame has a main CSS node with name frame. This node can be given the
- * style class .flat, which disables drawing of the border, equivalent to
- * calling gtk_frame_set_shadow_type() with GTK_SHADOW_NONE.
+ * style class .flat, which is used by themes to disable drawing of the border.
+ * Another way to do this in code is to call gtk_frame_set_shadow_type() with
+ * GTK_SHADOW_NONE to add the .flat class or any other shadow type to remove it.
  */
 
 struct _GtkFramePrivate