Also, for people interesed in contributing to Luau, the entire internal Luau API is available online for perusing (also distributed with the Luau software distribution under docs/api/). If you're only interested in using the libuau library (the C interface to Luau's functionality), you'll only be interested in the libuau.h Interface
Please note that, as always, all documentation is a work-in-progress. I've attempted to provide a rather thorough overview of all pertinent information through the Whitepaper and through doxygen code comments, but much is still lacking (particularly, the Programmer's Guide and Contributor's Guide have not yet been written for the whitepaper). Don't expect these documents to be complete — however, if you find a particularly important piece of information missing, find any section confusing, or even just find a spelling or grammar mistake, please contact me and I'll try to amend the problems.