Improve documentation on the structure of the survey file

[jonathanlking]

At Zurihac @nh2 graciously explained how the survey/default.nix worked. Hopefully some of the notes I made will be useful to others!

[jonathanlking]

@nh2 apologies this has taken so long! I think this an outline of what I wrote down, but I can flesh out the sections more if necessary. I'm trying to decipher my notes on addStaticLinkerFlagsWithPkgconfig and remember exactly what it does and why. (Please correct me if I've got anything wrong/let me know if you're unhappy with anything!)