doc: add a sort by line length example

* doc/coreutils.texi (sort invocation): Add an example showing how
to sort data not directly supported by the sort command.

doc/coreutils.texi

@@ -4257,6 +4257,17 @@ by the sort operation.
 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
 @c @end example
 
+@item
+Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
+sort lines according to their length.
+
+@example
+awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
+@end example
+
+In general this technique can be used to sort data that the @command{sort}
+command does not support, or is inefficient at, sorting directly.
+
 @item
 Shuffle a list of directories, but preserve the order of files within
 each directory.  For instance, one could use this to generate a music