An understanding of READ_ONCE() and WRITE_ONCE() is important for kernel developers who will be dealing with any sort of concurrent access to data. So, naturally, they are almost entirely absent from the kernel's documentation.

More chuckles from the source:

   /*
    * Yes, this permits 64-bit accesses on 32-bit architectures. These will
    * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
    * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
    * (e.g. a virtual address) and a strong prevailing wind.
    */