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@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>