Documentation/x86: Document resctrl's new sparse_masks

The documentation mentions that non-contiguous bit masks are not
supported in Intel Cache Allocation Technology (CAT).

Update the documentation on how to determine if sparse bit masks are
allowed in L2 and L3 CAT.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Peter Newman <peternewman@google.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Tested-by: Peter Newman <peternewman@google.com>
Link: https://lore.kernel.org/r/3e9610997164f648e15c5c2e90d4944ce36504fe.1696934091.git.maciej.wieczor-retman@intel.com