[Info-vax] Disable hyper-threading from the DCL prompt

[John Reagan]

>From one of the authors of SET CPU /NOCOTHREAD

"what it does is to return one of the hyper threads to the loader; the loader then puts it into a semi-infinite loop where it does a (can't remember the pal call, but essentially a give up control to the other hyper thread).  It is not quite as good (in the sense of giving back all the cycles of the core to a single cpu) as turning hyper threads off entirely with HTHREADS or with the efi command CPUCONFIG.  However these later two require you to reboot once for HTHREADS and twice (well, two resets) for CPUCONFIG."

I'll make sure the help gets updated.