perf trace: Add usage of --no-syscalls in man page

perf trace supports --no-syscalls option but it's not listed in the man
page. (Though, I see an example using --no-syscalls in EXAMPLES
section.)

Committer note:

The --no-syscalls option tells 'perf trace' not to automagically ask for
raw_syscalls:sys_{enter,exit} to then format it in a strace like way.

This become more used as 'perf trace' got support for arbitrary events,
such as tracepoints, so more and more we use:

  # perf trace --no-syscalls -e nmi:*
     0.000 nmi:nmi_handler:perf_event_nmi_handler() delta_ns: 36649 handled: 1)
     0.019 nmi:nmi_handler:nmi_cpu_backtrace_handler() delta_ns: 2907 handled: 0)
     0.676 nmi:nmi_handler:perf_event_nmi_handler() delta_ns: 9401 handled: 1)
     0.680 nmi:nmi_handler:nmi_cpu_backtrace_handler() delta_ns: 288 handled: 0)
     0.701 nmi:nmi_handler:perf_event_nmi_handler() delta_ns: 4977 handled: 1)
     0.703 nmi:nmi_handler:nmi_cpu_backtrace_handler() delta_ns: 67 handled: 0)
     0.736 nmi:nmi_handler:perf_event_nmi_handler() delta_ns: 8549 handled: 1)
  ^C#
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexis Berlemont <alexis.berlemont@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1492063332-5745-1-git-send-email-ravi.bangoria@linux.vnet.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-trace.txt

@@ -123,7 +123,8 @@ the thread executes on the designated CPUs. Default is to monitor all CPUs.
 	major or all pagefaults. Default value is maj.
 
 --syscalls::
-	Trace system calls. This options is enabled by default.
+	Trace system calls. This options is enabled by default, disable with
+	--no-syscalls.
 
 --call-graph [mode,type,min[,limit],order[,key][,branch]]::
         Setup and enable call-graph (stack chain/backtrace) recording.