Add missing docs

src/Android/Avalonia.Android/AndroidPlatform.cs

@@ -25,6 +25,9 @@ namespace Avalonia
         }
     }
 
+    /// <summary>
+    /// Represents the rendering mode for platform graphics.
+    /// </summary>
     public enum AndroidRenderingMode
     {
         /// <summary>

src/Avalonia.X11/X11Platform.cs

@@ -220,6 +220,9 @@ namespace Avalonia.X11
 
 namespace Avalonia
 {
+    /// <summary>
+    /// Represents the rendering mode for platform graphics.
+    /// </summary>
     public enum X11RenderingMode
     {
         /// <summary>

src/Windows/Avalonia.Win32/Win32PlatformOptions.cs

@@ -4,6 +4,9 @@ using Avalonia.Platform;
 
 namespace Avalonia;
 
+/// <summary>
+/// Represents the rendering mode for platform graphics.
+/// </summary>
 public enum Win32RenderingMode
 {
     /// <summary>
@@ -22,6 +25,9 @@ public enum Win32RenderingMode
     Wgl = 3
 }
 
+/// <summary>
+/// Represents the Win32 window composition mode.
+/// </summary>
 public enum Win32CompositionMode
 {
     /// <summary>