[css-images-4] Copy over dithering paragraph from L3.

Author: fantasai

css-images-4/Overview.bs

@@ -1647,6 +1647,13 @@ Defining Gradient Color {#gradient-colors}
 	along with its <a>starting point</a> and <a>ending point</a>;
 	see above.)
 
+	Colors throughout the gradient field are then determined
+	by tying them to specific points along the [=gradient line=]
+	as specified by the gradient function.
+	UAs may “dither” gradient colors slightly
+	(randomly alternate individual pixels with nearby colors on the gradient line)
+	to effect a smoother gradient.
+
 <h4 id=color-stop-syntax>
 Color Stop Lists</h4>