Clarity the client behavior and clean the text


1) Replace the normative language in the section of Architecture Overview.
2) Clarify in the section of Client Behavior that the Enable option and Server Address option MUST be requested in ORO in every Solicit, Request, Renew and Information-request messages, and the client MUST NOT request the two options in Boot-request-v6 message. 
3) Add a reference to RFC4242 to handle stateless DHCPv6 configuration.
4) Rename "DHCP client" to "4o6 DHCP Client", "4o6 Server" to "4o6 DHCP Server" and "4o6 Servers Address Option" to "4o6 Server Address Option".
5) Add some text in Security Considerations to discuss issues about multiple addresses inside Server Address Option.
6) Clean up the text and correct some typos.