document the mysterious abc part of the header

One can't help but wonder what it is about. Disclaimer: I'm far from being a specialist, I barely know varnish. This is what I understand after a cursory reading of [this document](http://www.w3.org/TR/edge-arch). Please check this carefully.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | none
Done and rebased on master (I made the PR on master, not sure why, tell me if you would like me to close it and make another on 2.3)