Cleanup some text for the X-Headers

All,
Re-reading the document this afternoon, as we get ready to release TAXII 2.1 CSD01, I noticed that there was some ambiguous text in section 3.2 for the X-Headers. Please review the text clarifications inline.
https://docs.google.com/document/d/1EsiWY7TGqt9yH6QUXv4c-opXSr3wR0TDMt8Q0yJjpoo/edit#heading=h.vsvl27q20vdp
Bret