bug#30177: improve shred manpage

[devzero]

i tried to compare performance of shred and dd, because a colleague told, shred 
-n0 -z would be faster.

the shred manpage tells:
If FILE is -, shred standard output 

But:
# shred - | pv >/dev/null
shred: - invalid file type
0 B 0:00:00 [   0 B/s]  [<=>

i found https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=155175 

as "-" works when redirecting to a file, i assume it doesn't work with a pipe 
because shred needs to seek within the file, which is not possible with a pipe.

i think shred -n0 -z  could be handled as a special case but it would not be 
worth the effort.

could you perhaps enhance the manpage apropriately, as already suggested in the 
debian bugreport?

regards 
roland