Improve language around backups before an upgrade

In the "Preparing for an upgrade section" of [this page](https://www.elastic.co/guide/en/elastic-stack/master/upgrading-elastic-stack.html#_preparing_to_upgrade), we mention "Back up your data. You cannot roll back to an earlier version unless you have a backup of your data. For information about creating snapshots, see Snapshot and Restore."

Due to the incremental nature of snapshots, we may want to mention that a new repo must be created after upgrading indexes in 5.6 (and before the upgrade is performed). Otherwise, the user will need to perform a snapshot restore on a version of ES < 6.0.

One possibility for changing the language:

"Back up your data. You cannot roll back to an earlier version unless you have a backup of your data. **Note: snapshots are incremental, snapshot repos created in earlier versions of Elasticsearch may not be restorable in newer versions of Elasticsearch. For this reason, it is important to upgrade all indexes in 5.6, create a new snapshot repo, perform a snapshot, and then upgrade Elasticsearch.** For information about creating snapshots, see Snapshot and Restore."

CC @eskibars

打开 upgrading-elastic-stack.html 中的“Preparing for an upgrade”部分，并将其中关于备份的措辞与 Snapshot and Restore 文档进行比较。验证 5.6 索引升级和新存储库的顺序，然后更新警告，以便用户了解在升级 Elasticsearch 之前何时创建存储库和快照。

由索引模型根据 Issue 内容生成。