docs: ABI: sysfs-bus-event_source-devices: Document sysfs caps entry for PMU

Details is added about "caps" attribute group in the ABI documentation.
This is used to expose some of the PMU attributes in "caps"
directory under : /sys/bus/event_source/devices/<dev>/. The dev/caps
will contain information about features that platform specific PMU
supports.
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220520084630.15181-2-atrajeev@linux.vnet.ibm.com

Documentation/ABI/testing/sysfs-bus-event_source-devices-caps

+What:		/sys/bus/event_source/devices/<dev>/caps
+Date:		May 2022
+KernelVersion:	5.19
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		Attribute group to describe the capabilities exposed
+		for a particular pmu. Each attribute of this group can
+		expose information specific to a PMU, say pmu_name, so that
+		userspace can understand some of the feature which the
+		platform specific PMU supports.
+
+		One of the example available capability in supported platform
+		like Intel is pmu_name, which exposes underlying CPU name known
+		to the PMU driver.
+
+		Example output in powerpc:
+		grep . /sys/bus/event_source/devices/cpu/caps/*
+		/sys/bus/event_source/devices/cpu/caps/pmu_name:POWER9