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 <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Taeung Song <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/tips.txt

···
 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
+Show current config key-value pairs: perf config --list
+Show user configuration overrides: perf config --user --list