Update Monitor Persistence Docs with Async Specifics

[TheBlueMatt]

In #1106 various suggestions for clearer docs came up repeatedly, most of which depended on us actually properly supporting fully async persistence, which was previously considered unsafe. Once we do support fully async persistence, we should go back through those suggestions and, yet again, rewrite all the docs on the persistence status enum and chain::Watch.

[TheBlueMatt]

Note from #1792 - make sure the docs we end up with for fully-async monitor persistence mention that updates must be persisted in the order they're generated in.