not *all* retags might be explicit in Runtime MIR
In https://github.com/rust-lang/rust/pull/105317 I made Miri treat `Rvalue::Ref/AddrOf` as implicit retagging sites. This updates the MIR docs accordingly.
For `Rvalue::Ref` I think this makes a lot more sense: creating a new reference is their entire point, so we can avoid bloating the MIR with retags. Also this seems to be the best way to handle cases like `*ptr = &[mut] ...`, where doing a retag is somewhat questionable since maybe `*ptr` points to another place now?
For `Rvalue::AddrOf`, Stacked Borrows needs this because even raw ptrs need some retagging, but Tree Borrows doesn't do ant retagging here and I hope we'll end up with a model where raw pointers don't get retagged.
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