x86/topology: Document cpu_llc_id

It means different things on Intel and AMD so write it down so that
there's no confusion.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yazen Ghannam <yazen.ghannam@amd.com>
Link: http://lkml.kernel.org/r/20161117094557.jm6hwzdd52h7iwnj@pd.tnic
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/x86/topology.txt

@@ -63,6 +63,15 @@ The topology of a system is described in the units of:
     The maximum possible number of packages in the system. Helpful for per
     package facilities to preallocate per package information.
 
+  - cpu_llc_id:
+
+    A per-CPU variable containing:
+    - On Intel, the first APIC ID of the list of CPUs sharing the Last Level
+    Cache
+
+    - On AMD, the Node ID or Core Complex ID containing the Last Level
+    Cache. In general, it is a number identifying an LLC uniquely on the
+    system.
 
 * Cores: