perf report/top: Add a tip about system-wide collection from all CPUs

Signed-off-by: Donghyun Kim <dongdong9335@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Taeung Song <taeung@kosslab.kr>
Link: http://lkml.kernel.org/r/1475187357-21882-1-git-send-email-dongdong9335@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>