While discussing `Span<T>` safety rules with the F# team I realized we
hadn't fully documented our dependency on the shape of the `Span<T>`
API. Added that plus some of the future considerations we discussed for
`Span<T>`.
https://github.com/Microsoft/visualfsharp/pull/4888
We no longer assume that ` Span<T> span = new Span<T>(ref variable); ` can actually "wrap" the reference to the variable.
This allows to relax some of the safety requirements.