Please review

We have added suggested text to resolve the following github issues, please review, comment, and improve as needed:
Issue 8:  max_content_length example value is not exemplary
https://github.com/oasis-tcs/cti-taxii2/issues/8
Found in two places in 4.2.1
Issue 17:  Add text about TLS 1.3 0-rtt
https://github.com/oasis-tcs/cti-taxii2/issues/17
Found in 8.2.2
Issue 19:  The X headers are not clear about which date
https://github.com/oasis-tcs/cti-taxii2/issues/19
Found in 3.2 and 5.6.1
Thanks
Bret