Explain stacked layers of keep alive protocols

[tfpauly]

Closes #1428

[gorryfair]

I think this follows what we talked about. I also think it is a very valuable example of how the stack deals with a property that might inform multiple layers - saying that a specific property does not infer that the implementation makes the same request at each layer. Thanks for doing this.

[abrunstrom]

thanks, looks good! made a small comment.

[mwelzl]

This is really great, in my opinion, and appears to say exactly what needs to be said. It makes me wonder in retrospect why I couldn't wrap my head this.