Discourage use of GetResult.expiry() in favor of alternative that returns Instant

The Duration returned by GetResult.expiry() is bogus – it represents the time span between the start of the epoch and the instant when the document expires.

The return type of this method should really be `Instant`, but that would be a breaking change.

Instead, we can discourage using the `expiry()` method and add an `expiryTime()` method that returns an Instant.

We plan to deprecate `expiry()` method in the next minor release, when we can remove the "uncommitted" tag from `expiryTime()`.