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

tools/perf/builtin-record.c

@@ -1989,8 +1989,8 @@ static struct option __record_options[] = {
 	OPT_BOOLEAN(0, "timestamp-boundary", &record.timestamp_boundary,
 		    "Record timestamp boundary (time of first/last samples)"),
 	OPT_STRING_OPTARG_SET(0, "switch-output", &record.switch_output.str,
-			  &record.switch_output.set, "signal,size,time",
-			  "Switch output when receive SIGUSR2 or cross size,time threshold",
+			  &record.switch_output.set, "signal or size[BKMG] or time[smhd]",
+			  "Switch output when receiving SIGUSR2 (signal) or cross a size or time threshold",
 			  "signal"),
 	OPT_INTEGER(0, "switch-max-files", &record.switch_output.num_files,
 		   "Limit number of switch output generated files"),