Re-work backup restore docs to avoid user error

RE [this EAR](https://github.com/influxdata/EAR/issues/3368)

This [doc page](https://docs.influxdata.com/enterprise_influxdb/v1.9/administration/backup-and-restore/#restore-from-an-incremental-backup-for-a-database-and-merge-that-database-into-an-existing-database) led the user to a poor practice of querying for their full dataset. This doc should emphasize using `influx_inspect export` with a time bound equal to `now() - `.

If we want to include the `SELECT INTO` method, we should call out that the data set should be small...and that there should, again, be a time bound on that query (like the export above).