bug#70532: sort: Mention counting fields from the end

[Dan Jacobson]

In (info "(coreutils) sort invocation") be sure to add an example of a
way or workaround for counting fields from the end of the line. E.g., we
want to sort on the last field, but don't know for sure how many fields
a line might contain. E.g., sort by surname, when lines consist of First
[Middle...] Surname. perl -a uses $F[-1]. so maybe sort(1) could also
use a negative field number. Same for character number.

sort (GNU coreutils) 9.4