Auto merge of #7856 - Manishearth:impl-safety, r=xFrednet

missing_safety_doc: Handle 'implementation safety' headers as well

We hit some FPs on this in `yoke`, it's somewhat normal to mark trait impl safety with "implementation safety". We could also broaden the check for headers which contain the word "safety" somehow, or split out impl safety stuff to only apply to traits.

changelog: handle 'implementation safety' headers in `missing_safety_doc`