perf stat: Update help text

<command>...::

	Any command you can specify in a shell.

-e::

--event=::

                             0:0: cpu-cycles          

                             1:4: migrations          

                           rNNN: raw PMU events (eventsel+umask)

-i::

--inherit::

        child tasks inherit counters

-p::

--pid=<pid>::

        stat events on existing pid

-a::

        system-wide collection

EXAMPLES

--------

$ perf stat sleep 1

$ perf stat -- make -j

 Performance counter stats for 'sleep':

 Performance counter stats for 'make -j':

       0.678356  task clock ticks     (msecs)

              7  context switches     (events)

              4  CPU migrations       (events)

            232  pagefaults           (events)

        1810403  CPU cycles           (events)

         946759  instructions         (events)

          18952  cache references     (events)

           4885  cache misses         (events)

    8117.370256  task clock ticks     #      11.281 CPU utilization factor

            678  context switches     #       0.000 M/sec

            133  CPU migrations       #       0.000 M/sec

         235724  pagefaults           #       0.029 M/sec

    24821162526  CPU cycles           #    3057.784 M/sec

    18687303457  instructions         #    2302.138 M/sec

      172158895  cache references     #      21.209 M/sec

       27075259  cache misses         #       3.335 M/sec

 Wall-clock time elapsed:  1001.252894 msecs

 Wall-clock time elapsed:   719.554352 msecs

SEE ALSO

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