Update for new Angle type, @@ operator

I think this has all necessary changes to the manual.  I grepped for occurrences of 'turn', 'rad', 'deg', and 'angle', and fixed each.

However, the Haddock links from the manual are broken.  Links for `Angle` and related functions point to `Diagrams.ThreeD`, where I export them, rather than to `Diagrams.TwoD.Types` where they are defined.  Is there a sensible way to make the links go the right place, or should I stop reexporting them for now?