Documentation: tracing: Add per-group/all events enablement desciption

Add ftrace.event.<GROUP>.enable and ftrace.event.enable bootconfig
description in the boot-time tracing document.

Link: https://lkml.kernel.org/r/162264438901.302580.10697703336929432947.stgit@devnote2

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