Update IANA section

[csosto-pk]

From Sean T:

Also, you’re going to need to update the IANA considerations to include the other values needed in the registry:

• Value, which is a number between 0 and 2039.
  ** Just don’t pick 8 since that’s already been used.
• Flag Name, which is a string
• Message, which like the "TLS 1.3" field in the ExtensionType
  registry contains the abbreviations of the messages that may
  contain the flag: CH, SH, EE, etc.
• Recommended, which is a Y/N value determined in the document
  defining the optional feature.
• Reference, which is a link to the document defining this flag.