perf tools: Document relation of per-thread event count feature

perf tools: Document relation of per-thread event count feature

The 'perf record -s' and 'perf report -T' should be used together to see
per-thread event counts.  Document the relation of these commands.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1431184784-30525-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>