Use pbkdf2 for backup key derivation

Fixes a warning thrown by OpenSSL during backup runs, and makes it more
secure.

doc/backups.md

@@ -12,4 +12,10 @@ The S3 credentials as well as the backup password are stored in the
 
 To decrypt a backup archive, use the following command:
 
+    openssl aes-256-cbc -d -base64 -pbkdf2 -in my_backup.tar.enc -out my_backup.tar
+
+If you get an error message along the lines of "bad decrypt", the archive was
+likely encrypted before we switched the key derivation scheme. Try without
+`-pbkdf2` in this case:
+
     openssl aes-256-cbc -d -base64 -in my_backup.tar.enc -out my_backup.tar