Improve the description of how runtime dependencies are found

[noamraph]

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I find the issue of how Nix decides on dependencies to be quite mysterious. I wasn't sure what exactly was meant with the current description in chapter 9. So I did some searching, and some experimentation, and got to the algorithm that I wrote in this PR.

What do you think?

I would really like it if someone verifies that I'm right. I couldn't find any authoritative reference on the exact algorithm, and a lot of references to this pill, so what will be written here will probably be used as a reference by others.