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

tools/perf/Documentation/perf-record.txt

@@ -45,6 +45,14 @@ OPTIONS
           param1 and param2 are defined as formats for the PMU in:
           /sys/bus/event_sources/devices/<pmu>/format/*
 
+	  There are also some params which are not defined in .../<pmu>/format/*.
+	  These params can be used to set event defaults.
+	  Here is a list of the params.
+	  - 'period': Set event sampling period
+
+	  Note: If user explicitly sets options which conflict with the params,
+	  the value set by the params will be overridden.
+
         - a hardware breakpoint event in the form of '\mem:addr[/len][:access]'
           where addr is the address in memory you want to break in.
           Access is the memory access type (read, write, execute) it can