Add documentation links to some functions

[thejoshwolfe]

These are only about 90% of the story, but it's a lot better than nothing. Is this even the right source of information? There are quite a few discrepancies, but it's mostly really similar.

This PR includes #7 optimistically, but I can easily separate it if you'd like.

[rfvgyhn]

Yeah, this looks good. We can also include links to the factorio data repo. Specifically map-settings and map-gen-settings.

As for the discrepancies, there's a decent chance this project doesn't implement everything correctly. When I first made this, I started from someone else's work and tweaked things until they looked OK for the specific map I was hosting at the time. Didn't think others would ever use it.

[thejoshwolfe]

Didn't think others would ever use it.

Well, I didn't use the code as-is. I ported it to python here: https://github.com/thejoshwolfe/Archipelago/blob/space-age/worlds/factorio_space_age/MapExchangeString.py

My use case is slightly different. I want to read and write the exchange string format; I don't need to support older factorio versions; and I really only need a small part of it to be named accurately (seed, no enemies mode, pollution enabled). If a block of fields in the middle is shifted, it won't even be observable for me, because a round trip re-encode would put it all back to its correct place.