Documentation: tracing: Add tracing_on option to boot-time tracer

Add tracing_on option description to the boot-time tracer.

Link: https://lkml.kernel.org/r/159978523520.485820.9250337223076929279.stgit@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/boottime-trace.rst

@@ -61,6 +61,10 @@ These options can be used for each instance including global ftrace node.
 ftrace.[instance.INSTANCE.]options = OPT1[, OPT2[...]]
    Enable given ftrace options.
 
+ftrace.[instance.INSTANCE.]tracing_on = 0|1
+   Enable/Disable tracing on this instance when starting boot-time tracing.
+   (you can enable it by the "traceon" event trigger action)
+
 ftrace.[instance.INSTANCE.]trace_clock = CLOCK
    Set given CLOCK to ftrace's trace_clock.