addition to the documentation

[J]

On 17-01-29 15:15:35, Cameron Simpson wrote:
> On 28Jan2017 12:54, J <mailinglists at 927589452.de> wrote:
> > wanted to add the following small explanation to the backup section:
> > ZFS [...]
> > #########
> > #!/usr/bin/env bash
> 
> Please, just:
> 
>  #!/bin/sh
> 
> There's nothing there that needs bash, and _all_ UNIX systems have a
> /bin/sh.  Portable.
> 

I did just fix the header without looking at the real script part,
but i tried changing to sh, and it breaks;
it could probably be fixed with a rewrite of the case statements in
sh-syntax

Greets
J