Write trait requirements in documentation.

I've not done any generated HTML in the past so please point out any formatting issues.
Just seeing this in the https://github.com/OpenRA/OpenRA/wiki/Traits/_compare/d3a6bbdc2fcc41c4f0227be0b83f811c86277c5c...159257ce3810a7e358359c899b42eebd12cf3554 diff. `doc.AppendLine("\n");` is saying the same thing twice over as `AppendLine` is already adding a newline just like `Write` vs `WriteLine`. This should be `doc.AppendLine();` instead, but that is not too serious. Otherwise looking great and really useful for modders.