perf config: Document variables for 'report' section in man page

Explain 'report' section's variables:

  'percent-limit', 'queue-size' and 'children'.

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/1454577913-16401-4-git-send-email-treeze.taeung@gmail.com


[ Fix some grammar issues, add some more info ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>