TL;DR As we do for `absolute::LockTime` and for the same reasons;
implement `ArbitraryOrd` for `relative::LockTime`.
locktimes do not have a semantic ordering if they differ (blocks, time)
so we do not derive `Ord` however it is useful for downstream to be able
to order structs that contain lock times. This is exactly what the
`ArbitraryOrd` trait is for.
Update the rustdocs in `relative` and mirror the docs changes in
`absolute`.
Fix: #2566
3520f550f0