It’s a 404! → http://unity3d.com/support/documentation/Manual/Example:%20Creating%20a%20Billboard%20Plane.html
You get linked to it from here: http://unity3d.com/support/documentation/Manual/Generating%20Mesh%20Geometry%20Procedurally.html
and here: http://unity3d.com/support/documentation/Manual/Using%20the%20Mesh%20Class.html
Hope thats enough details to get it fixed
EDIT: Does anyone have a link to the page thats missing at all?