Indicate in rustdoc that items are conditionally compiled based on Rust toolchain version

## Progress

- [x] Implement on v0.8.x - https://github.com/google/zerocopy/pull/1944
- [x] ~~Back-port [#1944](https://github.com/google/zerocopy/pull/1944) to main~~
  - Using the `needs-backport` label instead