The system call markdown documentation is generated from the syscall definitions in
//zircon/vdso by the script
//zircon/scripts/update-docs-from-fidl. This is done manually and the generated results are checked in.
The system call definitions are generated by kazoo which produces output for both the kernel and userspace in a variety of languages. This output includes C or C++ headers for both the kernel and userspace, syscall entry points, other language bindings, and so on. This tool is invoked as a part of the build, rather than checking in its output.