jot.1: Explain default argument values more precisely

The way jot(1) defaults missing arguments doesn't match the behaviour
described in the manpage, which states that with fewer than 3 arguments
missing values are supplied from left to right.

In fact, with one or two arguments, the last (s which is step size or seed)
defaults to 1 (or -1 if begin and end specify a descending range), and then
omitted arguments are set to default starting with the leftmost until three
arguments are available.

This is why jot 2 1000 prints 1000 and 1001 instead of 1000 and 100.

PR: 135475
Submitted by: Jonathan McKeown <j.mckeown@ru.ac.za>
Approved by: doc (bcr)
Differential Revision: https://reviews.freebsd.org/D21736
Event: EuroBSDcon 2019