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

tools/perf/Documentation/tips.txt

@@ -28,3 +28,4 @@ To change sampling frequency to 100 Hz: perf record -F 100
 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