Followup to #126761 and #124482. rustdoc-json should be extended to support [RFC 3484](https://rust-lang.github.io/rfcs/3484-unsafe-extern-blocks.html). This also intersects the [discussion of supporting unstable lang features in rustdoc-JSON](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Rustdoc.20JSON.3A.20Experimental.20rustc.20features), which has no decision yet.
Followup to #126761 and #124482. rustdoc-json should be extended to support RFC 3484.
This also intersects the discussion of supporting unstable lang features in rustdoc-JSON, which has no decision yet.