perf top/report: Add tips about a list option

Add two tips that describe --list option of config sub-command and
explain how to choose particular config file location.

Signed-off-by: Nambong Ha <over3025@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/1475191562-3240-1-git-send-email-over3025@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>