Improve "known problems" of `interior_mutable_key`
Improve "known problems" of `interior_mutable_key`

* Remove the mention to `Rc` and `Arc` as these are `Freeze`
  so the lint correctly handles already.

* Instead, explain what could cause a false positive,
  and mention `bytes` as an example.

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