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

Open the “Preparing for an upgrade” section in upgrading-elastic-stack.html and compare its backup wording with the Snapshot and Restore documentation. Verify the 5.6 index-upgrade and new-repository sequence, then update the warning so users know when to create the repository and snapshot before upgrading Elasticsearch.

Written by the indexing model from the issue text.