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>