Rollup merge of #152613 - RalfJung:unsafe-keyword-docs, r=traviscross

unsafe keyword docs: bring back unsafe_op_in_unsafe_fn lint discussion

@traviscross in rust-lang/rust#141471 you asked me to also update the text to account for the edition change. Apparently I did that by entirely removing this part of the discussion (except for a dangling forward reference, a "see below"). Given that old editions still exist and given that `unsafe_op_in_unsafe_fn` is just a lint so the old behavior also still exists on new editions, I am no longer sure that was a good idea, so this brings back the old text with some editing to explain the current situation.