fs/resctrl: Emphasize that L3 monitoring resource is required for summing domains

The feature to sum event data across multiple domains supports systems with
Sub-NUMA Cluster (SNC) mode enabled. The top-level monitoring files in each
"mon_L3_XX" directory provide the sum of data across all SNC nodes sharing an
L3 cache instance while the "mon_sub_L3_YY" sub-directories provide the event
data of the individual nodes.

SNC is only associated with the L3 resource and domains and as a result the
flow handling the sum of event data implicitly assumes it is working with
the L3 resource and domains.

Reading of telemetry events does not require to sum event data so this feature
can remain dedicated to SNC and keep the implicit assumption of working with
the L3 resource and domains.

Add a WARN to where the implicit assumption of working with the L3 resource
is made and add comments on how the structure controlling the event sum
feature is used.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lore.kernel.org/20251217172121.12030-1-tony.luck@intel.com

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