perf record: Document --group option

The 'perf record --group' option lacks documentation and confuses users.
As -e/--event option already supports group spec, it should not be used
anymore.

Also add a short description of event group itself.
Reported-by: NStephane Eranian <eranian@google.com>
Signed-off-by: NNamhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1425266013-5034-1-git-send-email-namhyung@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-record.txt

@@ -55,6 +55,11 @@ OPTIONS
           If you want to profile write accesses in [0x1000~1008), just set
           'mem:0x1000/8:w'.
 
+	- a group of events surrounded by a pair of brace ("{event1,event2,...}").
+	  Each event is separated by commas and the group should be quoted to
+	  prevent the shell interpretation.  You also need to use --group on
+	  "perf report" to view group events together.
+
 --filter=<filter>::
         Event filter.
 
@@ -104,6 +109,10 @@ OPTIONS
 	specification with appended unit character - B/K/M/G. The
 	size is rounded up to have nearest pages power of two value.
 
+--group::
+	Put all events in a single event group.  This precedes the --event
+	option and remains only for backward compatibility.  See --event.
+
 -g::
 	Enables call-graph (stack chain/backtrace) recording.