bug#25130: say something about competing ls options

[積丹尼 Dan Jacobson]

At the bottom of
(info "(coreutils) Sorting the output")
mention something like:

===Competing options===
Sometimes one gives "competing options" to ls,
e.g., ls -lXS

Well it turns out in the case of ls -lXS or ls -lSX, one will always
get...

There is no way to sort first via X and then via S within...,
or first via S and then via X within...



Please do mention something, else one gets wild dreams.