Patch.

Fixing comments, normalize comments, added comments.
Note - I did not touch GraphicsDevice hardly cuz I do not understand rules and your plans about paragraphs or references to unmanaged code.
Overall :
+ refresh Utilities.cs
+ refresh Rectangle.cs
+ refresh GraphicsDevice.cs
+ refresh Effect.cs
+ added few methods for ViewportF which validate ViewportF through d3d viewport rules.
+ fix issue 371
+ refresh MathUtil.cs - added possibility of using extension methods in MathUtil on NET2.0 platform.