bug#31055: Document ls -Ur -fr

[積丹尼 Dan Jacobson]

(info "(coreutils) Sorting the output") says

‘-r’
‘--reverse’
     Reverse whatever the sorting method is—e.g., list files in reverse
     alphabetical order, youngest first, smallest first, or whatever.

OK but mention 'whatever' doesn't mean -U, -f, because they are sorted
in "unsorted" order. In fact perhaps trigger an error.

Actually directory order is still an order, so in fact it is odd that -r
is "broken" here.