doc: reference shuf(1) from the sort (-R) man page

* man/sort.x: Cross reference with shuf(1).
* src/sort.c (usage): Mention shuf(1) with -R option.
Suggested in http://bugs.debian.org/641166

man/sort.x

@@ -3,4 +3,4 @@ sort \- sort lines of text files
 [DESCRIPTION]
 .\" Add any additional description here
 [SEE ALSO]
-uniq(1)
+shuf(1), uniq(1)

src/sort.c

@@ -451,7 +451,7 @@ Ordering options:\n\
 "), stdout);
       fputs (_("\
   -n, --numeric-sort          compare according to string numerical value\n\
-  -R, --random-sort           sort by random hash of keys\n\
+  -R, --random-sort           sort by random hash of keys.  See also shuf(1)\n\
       --random-source=FILE    get random bytes from FILE\n\
   -r, --reverse               reverse the result of comparisons\n\
 "), stdout);