Remove text of using allocated v4 to another interface & some corrections

1) Remove the last para of sec 8, because it conflicts dhcp RFC.
2) some corrections 
3) For your convenience, put Marcin's change log here:
- use Boot-request-v6 and Boot-reply-v6 instead of BOOTREQUESTV6 etc
- Changed Boot-reply-v6 and Boot-request-v6 message to support flags
- Described use of Unicast flag in Boot-request-v6 message
- Minor corrections.