Auto merge of #6019 - rail-rain:improve_known_probs_mut_key, r=flip1995
Auto merge of #6019 - rail-rain:improve_known_probs_mut_key, r=flip1995

Improve the "known problems" section of `interior_mutable_key`

* Remove the mention to `Rc` and `Arc` as these are `Freeze` (despite my intuition) so the lint correctly handles already.
* Instead, explain what could cause a false positive, and mention `bytes` as an example.

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changelog: Improved the "known problems" section of `interior_mutable_key`

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