For the Apple II, it was pure folklore, information passed on from developer to developer. Sample code is OK for docs. To be a platform at a theoretic level, you can stop there. Even an undocumented API with no developers, no users, and no potential is a platform. But, to be interesting, a platform must have other things.
Users are very important.
Lots of them. People who are doing cool things. Sometimes popular software or hardware doesn't become a platform until lots of people use it. Lotus published APIs for its spreadsheet long after the product became popular, for example. The Lotus platform had an installed base of users right from Day One.