x86/resctrl: Update documentation with Sub-NUMA cluster changes

With Sub-NUMA Cluster mode enabled the scope of monitoring resources is
per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in
their name refer to Sub-NUMA nodes instead of L3 cache ids.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Peter Newman <peternewman@google.com>