Auto merge of #6090 - Fishrock123:rc_buffer-formatting-caveats, r=yaahc

lints: clarify rc_buffer and add caveats

This didn't display some types properly in the docs due the lack of code formatting.

Also, refs for the caveat:
https://github.com/rust-lang/rust-clippy/pull/6044#issuecomment-699559082
https://github.com/http-rs/surf/pull/242

Fwiw I can't get `cargo test` to run, even on nightly. I get:
```
error[E0463]: can't find crate for `rustc_ast`
```

*Please keep the line below*
changelog: none, nightly