Documentation: tracing: Add entry argument access at function exit

Add a notes about the entry argument access at function exit probes for
kprobes and fprobe trace event.

Link: https://lore.kernel.org/all/170952367549.229804.8843506960483577062.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>