Address IESG Review comments by 0529

1. Barry Leiba
1) Add the section of "Applicability" as Ted suggested (Section 4)
2) Update the first two paragraphs of the "DHCP 4o6 Client Behavior" section
2. Spencer Dawkins
Update the 2nd paragraph of "Relay Agent Behavior" section
3. Alissa Cooper
1) Change the option diagrams to have "option-code" on them.
2) Change "unknown message" draft to Normative reference, with a MUST in the text.
3) Editorial suggestions.
4. Stephen F. (not agreed yet)
1) "Applicability" section
2) Add an Informative reference to RFC951