lints: clarify rc_buffer and add caveats
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

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust