Code review policy documentation

Here's an updated version of the thing in yesterday's Etherpad.  It's clearer, and it also requires paired merges to be explicitly documented.

I still like the idea of linking to some examples, but a quick look over the examples didn't find any that showed the various sorts of documentation without having to click through an extra ... affordance, which I wouldn't expect contributors to think of doing.  So I'd like to leave that for the future.