SetProxy::mappedTo and Set docs

I realized I forgot to add `mappedTo` in a couple places, and I figured I'd brush up the docs while I was at it. 

- Adds `mappedTo` accessor to `Set`
- Adds `mappedTo` to `SetProxy` (I've been monkey-patching it to `AssociationSet`)
- Use metadoc format for Set docs
- Move SetProxy-related documentation to their new pages
- Add docs for `mappedTo`