man: Reword --management to prefer unix sockets over TCP

man: Reword --management to prefer unix sockets over TCP

It is more secure to use unix sockets instead of TCP ports for the
management interface, so reword it and provide some details why TCP is
not recommended.

Also re-arranged this section to be somewhat easier to read and clearer
on a few related details.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180228131918.12954-2-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16573.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ec100d7e4ce7aaeb731c22b0d86826bf295df6cd)