The online documentation can be found at:
http://dwachs.github.io/simutrans-sqapi-doc/
As a result of the discussion about possibly removing Squirrel from Extended, I came across this documentation for the first time.
Those documentation pages are awesome; I would even say beautiful. Very easy to follow even for someone who doesn't know much about object-oriented programming. Especially compared to the game code itself: it's entirely understandable that our generous volunteer coders want to get back to the game instead of explaining things for newbies, but it's great the scripting system is more accessible.
Thank you!