document or fix dangers of sort -u + any other option

[jidanni]

Please fix the third command,

$ echo -e 'a1\nb2\nc3\nb2'|sort -n|sort -u
a1
b2
c3
$ echo -e 'a1\nb2\nc3\nb2'|sort -u|sort -n
a1
b2
c3
$ echo -e 'a1\nb2\nc3\nb2'|sort -nu
a1

or warn right there on the man page.

Sure, you say

       -u, --unique
              with -c, check for strict ordering; without -c, output
              only the first of an equal run

but still warn that the above will happen if one tries to combine
anything with -u.