Auto merge of #103496 - petrochenkov:effvisdoc2, r=lqd

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 rust-lang/rust#102026, `is_exported` (or `is_reachable` in the worst case) is what you need.

Follow up to rust-lang/rust#98450.
r? `@GuillaumeGomez` `@lqd`