Improve documentation in ZooKeeperServer.superSecret

A security scan flagged the use of a hard-coded secret (ZooKeeperServer.superSecret) in conjunction with a java Random instance to generate a password:

byte[] generatePasswd(long id)

{             Random r = new Random(id ^ superSecret);             byte p[] = new byte[16];             r.nextBytes(p);             return p;     }

superSecret has the following javadoc:

 /**
   * This is the secret that we use to generate passwords, for the moment it
   * is more of a sanity check.
   */

It is unclear from this comment and looking at the code why it is not a security risk. It would be good to update the javadoc along the lines of "Using a hard-coded secret with Random to generate a password is not a security risk because the resulting passwords are used for X, Y, Z and not for authentication etc" or something would be very helpful for anyone else looking at the code.