maxRequestPathLength not in ASP.NET 4 documentation and doesn't work

Question

If I try to use the new maxRequestPathLength settings in an ASP.NET application it does not work. I get an unrecognized attribute error. I've tried using both ASP.NET Integrated and Classic application pools in IIS 7. What is also funny is that if you search for maxRequestPathLength on MSDN it is no where to be found in the documentation except in the list of new features for ASP.NET 4. What gives?