perf intel-pt: Document snapshot control command

The documentation describes snapshot mode.  Update it to include the new
snapshot control command.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/20200901093758.32293-7-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-intel-pt.txt

@@ -558,7 +558,7 @@ The mmap size and auxtrace mmap size are displayed if the -vv option is used e.g
 Intel PT modes of operation
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Intel PT can be used in 2 modes:
+Intel PT can be used in 3 modes:
 	full-trace mode
 	sample mode
 	snapshot mode
@@ -571,7 +571,8 @@ Sample mode attaches a Intel PT sample to other events e.g.
 
 	perf record --aux-sample -e intel_pt//u -e branch-misses:u
 
-Snapshot mode captures the available data when a signal is sent e.g.
+Snapshot mode captures the available data when a signal is sent or "snapshot"
+control command is issued. e.g. using a signal
 
 	perf record -v -e intel_pt//u -S ./loopy 1000000000 &
 	[1] 11435
@@ -582,7 +583,23 @@ Note that the signal sent is SIGUSR2.
 Note that "Recording AUX area tracing snapshot" is displayed because the -v
 option is used.
 
-The 2 modes cannot be used together.
+The advantage of using "snapshot" control command is that the access is
+controlled by access to a FIFO e.g.
+
+	$ mkfifo perf.control
+	$ mkfifo perf.ack
+	$ cat perf.ack &
+	[1] 15235
+	$ sudo ~/bin/perf record --control fifo:perf.control,perf.ack -S -e intel_pt//u -- sleep 60 &
+	[2] 15243
+	$ ps -e | grep perf
+	15244 pts/1    00:00:00 perf
+	$ kill -USR2 15244
+	bash: kill: (15244) - Operation not permitted
+	$ echo snapshot > perf.control
+	ack
+
+The 3 Intel PT modes of operation cannot be used together.
 
 
 Buffer handling