Documentation: {u,k}probes: add tracing_on before tracing

After following the document step by step, the `cat trace` can't be
worked without enabling tracing_on and might mislead newbies about
the functionality.
Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/kprobetrace.rst

@@ -189,6 +189,12 @@ events, you need to enable it.
   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
   echo 1 > /sys/kernel/debug/tracing/events/kprobes/myretprobe/enable
 
+Use the following command to start tracing in an interval.
+::
+    # echo 1 > tracing_on
+    Open something...
+    # echo 0 > tracing_on
+
 And you can see the traced information via /sys/kernel/debug/tracing/trace.
 ::
 

Documentation/trace/uprobetracer.rst

@@ -152,10 +152,15 @@ events, you need to enable it by::
 
     # echo 1 > events/uprobes/enable
 
-Lets disable the event after sleeping for some time.
+Lets start tracing, sleep for some time and stop tracing.
 ::
 
+    # echo 1 > tracing_on
     # sleep 20
+    # echo 0 > tracing_on
+
+Also, you can disable the event by::
+
     # echo 0 > events/uprobes/enable
 
 And you can see the traced information via /sys/kernel/debug/tracing/trace.