block: mention 'password-secret' option for -iscsi

block: mention 'password-secret' option for -iscsi

The 'password-secret' option was added

  commit b189346eb1784df95ed6fed610411dbf23d19e1f
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Thu Jan 21 14:19:21 2016 +0000

    iscsi: add support for getting CHAP password via QCryptoSecret API

but was not mentioned in the command line docs

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>