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@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/boottime-trace.rst

@@ -99,6 +99,12 @@ These options are setting per-event options.
 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.enable
    Enable GROUP:EVENT tracing.
 
+ftrace.[instance.INSTANCE.]event.GROUP.enable
+   Enable all event tracing within GROUP.
+
+ftrace.[instance.INSTANCE.]event.enable
+   Enable all event tracing.
+
 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.filter = FILTER
    Set FILTER rule to the GROUP:EVENT.