docs: ABI: sysfs-bus-nvdimm: Document sysfs event format entries for nvdimm pmu

Details are added for the event, cpumask and format attributes
in the ABI documentation.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@in.ibm.com>
Link: https://lore.kernel.org/r/20220225143024.47947-5-kjain@linux.ibm.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>