Document --encryption_key_file option in dgraph restore command. (#5646)

Author: martinmr

wiki/content/enterprise-features/index.md

@@ -179,6 +179,10 @@ backups built on top of it. Each time a new full backup is created, a new backup
 series with a different ID is started. The backup series ID is stored in each
 `manifest.json` file stored in every backup folder.
 
+The `--encryption_key_file` flag is required if you took the backup in an
+encrypted cluster and should point to the location of the same key used to
+run the cluster.
+
 The restore feature will create a cluster with as many groups as the original
 cluster had at the time of the last backup. For each group, `dgraph restore`
 creates a posting directory `p<N>` corresponding to the backup group ID. For