schedule: Rework documentation for schedule_add_entry

schedule: Rework documentation for schedule_add_entry

The previous documentation was very misleading and made
it hard to understand how the sigma parameter is used.

Rewrite it so that it better reflects the actual
implementation.

Change-Id: Idd79f7cbd38e8b0831f15866339c3677a367cd49
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1439
Message-Id: <20251215144656.17299-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35070.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>