perf record: Clarify help for --switch-output

The help description for --switch-output looks like there are multiple
comma separated fields. But it's actually a choice of different options.
Make it clear and less confusing.

Before:

  % perf record -h
  ...
          --switch-output[=<signal,size,time>]
                            Switch output when receive SIGUSR2 or cross size,time threshold

After:

  % perf record -h
  ...

          --switch-output[=<signal or size[BKMG] or time[smhd]>]
                            Switch output when receiving SIGUSR2 (signal) or cross a size or time threshold

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
LPU-Reference: 20190314225002.30108-4-andi@firstfloor.org
Link: https://lkml.kernel.org/n/tip-9yecyuha04nyg8toyd1b2pgi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>