[dnssd] Advertising proxy document updated in git

[Ted Lemon <[e-mail]>]

We had a good discussion about the advertising proxy document in our meeting on Monday (thanks to everyone who attended!). I promised to do a quick update based on the discussion, which wound up being a bit more work than I'd anticipated. However, I've gotten to a point where I think it's probably worth looking at, and so I've committed these changes to the github repo. I have not published a new version of the document yet. Hopefully Jonathan and Abtin can take a look at the document and see what they think.

To summarize what's changed:
- The document now talks specifically and only about the advertising proxy function, and not e.g. the SRP server, or the resolver.
- The document has a lot more detail about how to actually implement an advertising proxy
- I mentioned in San Francisco that it might make sense to use naming hierarchy of some sort to prevent conflicts between discrete namespaces when advertising them using mDNS, which would ordinarily flatten the namespace. This document contains two proposals for how to do this, which I think might actually work without requiring any mDNS changes.

I probably made other changes, but it's all a blur now, so please give it a read if you have time and let me know what you think.