documentation: avoid recommending --user nobody

documentation: avoid recommending --user nobody

Recommend to create an user dedicated to openvpn
so that there is no priviledge escalation between
different services using that user.

cf. https://wiki.ubuntu.com/nobody

Trac: #1335
CC: tincantech <tincantech@protonmail.com>
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20221129144731.35105-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25573.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>