Added API documentation for Mouse.cs and MouseState.cs.

Added API documentation for Mouse.cs and MouseState.cs based on http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.mouse.aspx and http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.input.mousestate.aspx.

Though I'm not sure if it's okay to base on MSDN docs.