Add documentation for alpha format.

src/Avalonia.Visuals/Platform/AlphaFormat.cs

@@ -1,9 +1,21 @@
 namespace Avalonia.Platform
 {
+    /// <summary>
+    /// Describes how to interpret the alpha component of a pixel.
+    /// </summary>
     public enum AlphaFormat
     {
+        /// <summary>
+        /// All pixels have their alpha premultiplied in their color components.
+        /// </summary>
         Premul,
+        /// <summary>
+        /// All pixels have their color components stored without any regard to the alpha. e.g. this is the default configuration for PNG images.
+        /// </summary>
         Unpremul,
+        /// <summary>
+        /// All pixels are stored as opaque.
+        /// </summary>
         Opaque
     }
 }