proxy-options.rst: Add proper documentation for --http-proxy-user-pass

proxy-options.rst: Add proper documentation for --http-proxy-user-pass

And extend examples section for authenticated HTTP proxies because
is was misleading.

Change-Id: I7a754d0b4a76a9227bf922f65176cd9ec4d7670c
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20240118164903.22519-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28083.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>