As alternative to reading the man page for a given application (with the caveat that the required file might be in a "*-doc.tcz" extension) I find myself heading over to
http://linux.die.net for a quick look there (e.g. finding
this one).
Mind you this approach can have the downside that the online man-page is for a different (e.g. older) version of the tool in question. But in most cases I've found what I was looking for.
With the information gained from reading the man page I'm pretty sure that a counter reset is not (yet) available via 'nload'. A bit of searching leads to articles like
this one, that suggest that one would have to un-install and re-install the driver for the NIC (plus reconfigure it) to achieve a resetting of the counters. There are other
pages, which indicate that this strategy might not work in all circumstances.
In my little test (with 'nload' running whilst dropping the driver) I saw that the reported value for 'Ttl:' jumped to '4.00 GBytes', which required a restart of 'nload' to properly recognize the reset counters. I therefore think that a rather simple amendment to 'nload' would be required to achieve the intended outcome.