- Mention the behavior for cross-crate re-exports
- Mention the behavior for linking to private items
- Require that disambiguators match, even if they're in the same
  namespace
- For the extension on associated items, require that traits be in scope
  to link to their associated items
- Add a possible `Deref` extension

This is not implemented and it's not clear it will be implemented.
Since this PR is only intended to match the current implementation,
I removed it from the RFC.

…ented.