[gdk-pixbuf] docs: Document the alpha blending function used

[Philip Withnall <pwithnall src gnome org>]

commit 0d51eb13643f8241393bbb4f4d905f3d4a7e4871
Author: Philip Withnall <philip tecnocode co uk>
Date:   Sat Jan 7 19:17:39 2017 +0000

    docs: Document the alpha blending function used
    
    https://bugzilla.gnome.org/show_bug.cgi?id=442452

 gdk-pixbuf/gdk-pixbuf-scale.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/gdk-pixbuf/gdk-pixbuf-scale.c b/gdk-pixbuf/gdk-pixbuf-scale.c
index cc41961..e6b78c2 100644
--- a/gdk-pixbuf/gdk-pixbuf-scale.c
+++ b/gdk-pixbuf/gdk-pixbuf-scale.c
@@ -58,6 +58,12 @@
  * and Sparc chipsets.  If desired, mediaLib support can be turned off by
  * setting the `GDK_DISABLE_MEDIALIB` environment variable.  
  * 
+ * The alpha blending function used is:
+ * |[
+ * Cd = Cs·As + Cd(1-As)
+ * ]|
+ * where `Cd` is the destination pixel color, `Cs` is the source pixel color,
+ * and `As` is the source pixel alpha.
  * 
  * The following example demonstrates handling an expose event by
  * rendering the appropriate area of a source image (which is scaled