documentation: improve documentation of --x509-track

documentation: improve documentation of --x509-track

In the current state it was completely unclear to me how you
would use this. Extended the description based on reading the
code and experimentation.

Change-Id: Ibf728f9d624e64ecda094d66fa562bd3916829d2
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20231213143324.226443-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27804.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>