rustc_metadata: Encode even less doc comments
The fact that `def_id` is in the `tcx.privacy_access_levels(())` table is not very meaningful, especially after https://github.com/rust-lang/rust/pull/102026, `is_exported` (or `is_reachable` in the worst case) is what you need.
Follow up to https://github.com/rust-lang/rust/pull/98450.
r? `@GuillaumeGomez` `@lqd`