tracing: Have the document reflect that the trace file keeps tracing enabled

Now that reading the trace file does not temporarly stop tracing while it is
open, update the document to reflect this fact.

Link: http://lkml.kernel.org/r/20200317213417.209675068@goodmis.org



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