Update documentation for SnapshotMergeReport.

    // Time from sys.boot_completed to merge start, in milliseconds.

    uint32 boot_complete_to_merge_start_time_ms = 8;

    // Merge failure code, filled if state == MergeFailed.

    // Merge failure code, filled if the merge failed at any time (regardless

    // of whether it succeeded at a later time).

    MergeFailureCode merge_failure_code = 9;

}