tracing/user_events: Document auto-cleanup and remove dyn_event refs

tracing/user_events: Document auto-cleanup and remove dyn_event refs

Now user_events auto-cleanup upon the last reference by default. This
makes it not possible to use the dynamics event file via tracefs.

Document that auto-cleanup is enabled by default and remove the refernce
to /sys/kernel/tracing/dynamic_events file to make this clear.

Link: https://lkml.kernel.org/r/20230614163336.5797-7-beaub@linux.microsoft.com
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>