powerpc: Fixing a cputhread code documentation

 * Note: This implementation is limited to a power of 2 number of

 * threads per core and the same number for each core in the system

 * (though it would work if some processors had less threads as long

 * as the CPU numbers are still allocated, just not brought offline).

 * as the CPU numbers are still allocated, just not brought online).

 *

 * However, the API allows for a different implementation in the future

 * if needed, as long as you only use the functions and not the variables