Style: Turn on "Assume Explicit For" and scope autolinks (#580)

By default, Bikeshed will resolve unscoped autolinks to definitions
_with_ a `for` value automatically if it can. Turning this option on
makes unscoped links only resolve to definitions _without_ a `for`,
i.e. top-level definitions. Since we want to scope links (already
called out in docs/SpecCodingConventions.md), turning this option on
reveals where we're not scoping links.

Docs: https://speced.github.io/bikeshed/#metadata-assume-explicit-for