perf report/top: Add a tip about source line numbers with overhead

There is a existing tip as below.

    If you have debuginfo enabled, try: perf report -s sym,srcline

However this tip only describe a condition to use --sort sym,scrline
options.  So there is lack of explanation in the tip. I think that it
would be better to add a tip that exactly explains the feature of --sort
srcline.

Signed-off-by: Seonyoung Kim <adamas0414@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/1475194602-5596-1-git-send-email-adamas0414@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>