perf record: Document setting '-e pmu/period=N/' in man page

The 'period' param is not defined in
/sys/bus/event_sources/devices/<pmu>/format/*, but can be used, document
it.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1436345097-11113-3-git-send-email-kan.liang@intel.com


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