tracing/user_events: Update documentation for ABI

The ABI for user_events has changed from mmap() based to remote writes.
Update the documentation to reflect these changes, add new section for
unregistering events since lifetime is now tied to tasks instead of
files.

Link: https://lkml.kernel.org/r/20230328235219.203-10-beaub@linux.microsoft.com



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