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.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/tips.txt

@@ -29,3 +29,4 @@ See assembly instructions with percentage: perf annotate <symbol>
 If you prefer Intel style assembly, try: perf annotate -M intel
 For hierarchical output, try: perf report --hierarchy
 Order by the overhead of source file name and line number: perf report -s srcline
+System-wide collection from all CPUs: perf record -a