The highlighted sentence doesn’t make too much sense for map_unchecked. It looks to me like the documentation was copy-pasted from map_unchecked_mut, where the “You must guarantee that […] you do not move out of the argument you receive to the interior function.” provision makes a lot more sense. For map_unchecked though, the &T argument to the mapped function doesn’t provide anything above what Pin<&T> already gave us, as Pin<&T> -> &T is safe via dereferencing or get_ref, anyways - so the unsafe part is merely about the return &U being wrapped into Pin<&U>.
Regarding your comment / example with RefCell, I cannot quite follow that. Note however that RefCell cannot soundly do structural pinning anyways, since a Pin<&RefCell<T>> allows mutable (non-pinned) access to the contained value via get_ref (which gives us the &RefCell<T>) and borrow_mut (which then gives &mut T), anyways.