tools/power turbostat: Document PMT in turbostat.8

Add a general description of the user interface for adding PMT
counters with the new --add pmt,... option.

Provide a complete example for requesting two counters.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>