Add description of default sorting behavior to zfs_list.8

The sorting logic is all in cmd/zfs/zfs_iter.c. I borrowed
where I could from the comments in the source code, but please
note that the comment to zfs_sort() is a little imprecise, or at
least incomplete, because it doesn't give any indication of the
chronological sort that will be used by default for snapshots in
zfs_compare().

While adding this description, I took the liberty to copy-edit
the rest of the file lightly.

In those edits, I've removed "If specified, you can list
property information by the absolute pathname or the relative
pathname" because, in context, it seems more confusing than
helpful.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Shawn Bayern <sbayern@law.fsu.edu>
Closes #15713
Closes #15869