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
I think we should turn this into a note, and mostly link to the docs on this, as I think most users won't need to worry about this (but I like the idea of showing people where they can find out more, I didn't know what the `abc` was before this either).:

```
.. note::

    The ``abc`` part of the header isn't important unless you have multiple "surrogates"
    that need to advertise their capabilities. See `Surrogate-Capability Header`_ for details.
```

Then at the bottom of the page:

```
.. _`Surrogate-Capability Header`: http://www.w3.org/TR/edge-arch
```