tracing/user_events: Document persist event flags

Users need to know how to make events persist now that we allow for
that. We also now allow the dynamic_events file to create events by
utilizing the persist flag during event register.

Add back in to documentation how /sys/kernel/tracing/dynamic_events can
be used to create persistent user_events. Add a section under registering
for the currently supported flags (USER_EVENT_REG_PERSIST) and the
required permissions. Add a note under deleting that deleting a
persistent event also requires sufficient permission.

Link: https://lkml.kernel.org/r/20230912180704.1284-4-beaub@linux.microsoft.com



Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>