[mutter] Update and fix comments

[Jasper St. Pierre <jstpierre src gnome org>]

commit e46c7fd27b27f1103f007a614a247dea404ebd33
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Thu Mar 14 17:55:24 2013 -0400

    Update and fix comments

 src/compositor/meta-window-group.h |   16 +++++++++-------
 src/meta/meta-background-group.h   |    5 +++--
 2 files changed, 12 insertions(+), 9 deletions(-)
---
diff --git a/src/compositor/meta-window-group.h b/src/compositor/meta-window-group.h
index 45dab55..1c3eebf 100644
--- a/src/compositor/meta-window-group.h
+++ b/src/compositor/meta-window-group.h
@@ -10,13 +10,15 @@
 /**
  * MetaWindowGroup:
  *
- * This class is a subclass of ClutterGroup with special handling for
- * MetaWindowActor when painting the group. When we are painting a stack
- * of 5-10 maximized windows, the standard bottom-to-top method of
- * drawing every actor results in a tremendous amount of overdraw
- * and can easily max out the available memory bandwidth on a low-end
- * graphics chipset. It's even worse if window textures are being accessed
- * over the AGP bus.
+ * This class is a subclass of ClutterActor with special handling for
+ * MetaWindowActor/MetaBackgroundActor/MetaBackgroundGroup when painting
+ * children.
+ *
+ * When we are painting a stack of 5-10 maximized windows, the
+ * standard bottom-to-top method of drawing every actor results in a
+ * tremendous amount of overdraw and can easily max out the available
+ * memory bandwidth on a low-end graphics chipset. It's even worse if
+ * window textures are being accessed over the AGP bus.
  *
  * The basic technique applied here is to do a pre-pass before painting
  * where we walk window from top to bottom and compute the visible area
diff --git a/src/meta/meta-background-group.h b/src/meta/meta-background-group.h
index 8c1cb4a..60dd490 100644
--- a/src/meta/meta-background-group.h
+++ b/src/meta/meta-background-group.h
@@ -9,8 +9,9 @@
  * MetaBackgroundGroup:
  *
  * This class is a subclass of ClutterActor with special handling for
- * MetaBackgroundActor when painting the group. It makes sure to only
- * draw the parts of the backgrounds not occluded by opaque windows.
+ * MetaBackgroundActor/MetaBackgroundGroup when painting children.
+ * It makes sure to only draw the parts of the backgrounds not
+ * occluded by opaque windows.
  *
  * See #MetaWindowGroup for more information behind the motivation,
  * and details on implementation.