Cleaned up the documentation on background clip and added an example.

doc-src/content/examples/compass/css3/background-clip.haml

@@ -0,0 +1,12 @@
+---
+title: Background Clip
+description: Background Clip In Action
+framework: compass
+stylesheet: compass/css3/_background_clip.sass
+example: true
+---
+- render "partials/example" do
+  %p
+    In this example, there's a red border with alpha transparency.
+    The padding box appears pink against the white background of the page.
+    The border box appears purple because it blends with the blue background instead.

doc-src/content/examples/compass/css3/background-clip/markup.haml

@@ -0,0 +1,5 @@
+.example
+  #padding-box
+    Padding Box
+  #border-box
+    Border Box

doc-src/content/examples/compass/css3/background-clip/stylesheet.sass

@@ -0,0 +1,15 @@
+@import compass/css3.sass
+.example
+  padding: 2em
+  div
+    background-color: blue
+    border: 10px solid rgba(255, 0, 0, 0.5)
+    color: white
+    +text-shadow(darken(#00f, 50%), 2px, 2px)
+    padding: 3px
+    text-align: center
+    margin-bottom: 2em
+  #padding-box
+    +background-clip("padding-box")
+  #border-box
+    +background-clip("border-box")

frameworks/compass/stylesheets/compass/css3/_background_clip.sass

@@ -1,15 +1,21 @@
 //
-  Clip the background (image and color) at the edge of the padding or border.
 
-  Default constant is padding-box, the box model used by modern browsers.
+  The default value is `padding-box` -- the box model used by modern browsers.
 
   If you wish to do so, you can override the default constant with `border-box`
 
-  To override to the border-box model, use this code:
+  To override to the default border-box model, use this code:
       !default_background_clip = "border-box"
 
 !default_background_clip ||= "padding-box"
 
+//
+  Clip the background (image and color) at the edge of the padding or border.
+
+  Legal Values:
+
+    * "padding-box"
+    * "border-box"
 =background-clip(!clip = !default_background_clip)
   // webkit and mozilla use the deprecated short [border | padding | content]
   !deprecated= "padding"