Update `ChannelMonitorUpdateStatus` documentation with async support

Update `ChannelMonitorUpdateStatus` documentation with async support

Since we now (almost) support async monitor update persistence, the
documentation on `ChannelMonitorUpdateStatus` can be updated to no
longer suggest users must keep a local copy that persists before
returning. However, because there are still a few remaining issues,
we note that async support is currently beta and explicily warn of
potential for funds-loss.

Fixes #1684