Update draft-XXX-dhc-dhcpv4-over-dhcpv6-00_0319.xml

1. Specify the client type can be different devices other than CPEs but in this doc CPE is taken as an example.
2. Specify that server uses link-address to identify the link client is attached to.
3. Specify that if server receives a DHCPv6 packet without the DHCPv4 Message Option, it should handle it as RFC 3315.