Actually, our documentation on MSDN has moved.
The URL for Microsoft adCenter API content on MSDN remains the same as it was before, http://msdn.microsoft.com/en-us/library/dd797073.aspx.
But if you navigate to the content from the MSDN library page, http://msdn.microsoft.com/library, you can now find adCenter API content via Online Services --> Microsoft Advertising Services. Previously the content was not as easily reached from the library page.
Walter Poupore Developer Content Manager Microsoft