Document how Pin != StableDeref

[CAD97]

Discussion on urlo.

TL;DR: Pin<P> fails two guarantees of StableDeref, even when Target: !Unpin: it does not guarantee that <P as Deref>::deref and <P as DerefMut>::deref_mut go to the same place, and it does not even guarantee that two invocations of <P as Deref>::deref go to the same place.

[Manishearth]

Could we just impl StableDeref for Pin<P> where P: StableDeref?

[Manishearth]

cc @Storyyeller

[Ten0]

I'm not sure how failing to satisfy these would break safety in crates like rental or owning_ref though. Any clue?
EDIT: Oh nevermind I found: if Deref points to somewhere that moves when the struct moves (e.g. &self.some_field), it breaks. Pin seems to not guarantee that is not the case.