perf data: Update documentation section on cpu topology

Add an explanation of each cpu's core and socket identifier to the
perf.data file format documentation.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/20180528074433.16652-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf.data-file-format.txt

@@ -153,10 +153,18 @@ struct {
 	HEADER_CPU_TOPOLOGY = 13,
 
 String lists defining the core and CPU threads topology.
+The string lists are followed by a variable length array
+which contains core_id and socket_id of each cpu.
+The number of entries can be determined by the size of the
+section minus the sizes of both string lists.
 
 struct {
        struct perf_header_string_list cores; /* Variable length */
        struct perf_header_string_list threads; /* Variable length */
+       struct {
+	      uint32_t core_id;
+	      uint32_t socket_id;
+       } cpus[nr]; /* Variable length records */
 };
 
 Example: