perf config: Document variables for 'call-graph' section in man page

Explain 'call-graph' section and its variables:

  'record-mode', 'dump-size', 'print-type', 'order', 'sort-key',
  'threshold' and 'print-limit'.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1454577913-16401-3-git-send-email-treeze.taeung@gmail.com


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