document the mysterious abc part of the header

One can't help but wonder what it is about.
minor #3880 document the mysterious abc part of the header (greg0ire)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3880).

Discussion
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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

Commits
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48f55e2 document the mysterious abc part of the header