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

Documentation/ABI/testing/sysfs-bus-nvdimm

@@ -6,3 +6,38 @@ Description:
 
 The libnvdimm sub-system implements a common sysfs interface for
 platform nvdimm resources. See Documentation/driver-api/nvdimm/.
+
+What:           /sys/bus/event_source/devices/nmemX/format
+Date:           February 2022
+KernelVersion:  5.18
+Contact:        Kajol Jain <kjain@linux.ibm.com>
+Description:	(RO) Attribute group to describe the magic bits
+		that go into perf_event_attr.config for a particular pmu.
+		(See ABI/testing/sysfs-bus-event_source-devices-format).
+
+		Each attribute under this group defines a bit range of the
+		perf_event_attr.config. Supported attribute is listed
+		below::
+		  event  = "config:0-4"  - event ID
+
+		For example::
+			ctl_res_cnt = "event=0x1"
+
+What:           /sys/bus/event_source/devices/nmemX/events
+Date:           February 2022
+KernelVersion:  5.18
+Contact:        Kajol Jain <kjain@linux.ibm.com>
+Description:	(RO) Attribute group to describe performance monitoring events
+                for the nvdimm memory device. Each attribute in this group
+                describes a single performance monitoring event supported by
+                this nvdimm pmu.  The name of the file is the name of the event.
+                (See ABI/testing/sysfs-bus-event_source-devices-events). A
+                listing of the events supported by a given nvdimm provider type
+                can be found in Documentation/driver-api/nvdimm/$provider.
+
+What:          /sys/bus/event_source/devices/nmemX/cpumask
+Date:          February 2022
+KernelVersion:  5.18
+Contact:        Kajol Jain <kjain@linux.ibm.com>
+Description:	(RO) This sysfs file exposes the cpumask which is designated to
+		to retrieve nvdimm pmu event counter data.