Remove the FUD about random nonces

Now it's consistent with the documentation for public key encryption.

The NaCL pages also say random nonces are fine:
http://nacl.cr.yp.to/stream.html

The suggestion of per-person prefixes is not good either, since using two of the bytes for a prefix makes it orders of magnitude more likely that you get a collision.
I'd drop the sentence on birthday attacks, it's just confusing IMO.