Add keys 'num_auxio_threads' and 'num_nonio_threads' to "Setting Thread Allocations"

As of MB-49977 NonIO and AuxIO thread pool counts should now be set using the memcahed config rather than using the undocumented method of "extra_config_string" which will no longer work. This is to bring the thread pool counts inline with keys 'num_reader_threads' & 'num_writer_threads'. So we should update the documentation Setting Thread Allocations to reflect this.