Documentation: probes: Add a new ret_ip callback parameter

Add a new ret_ip callback parameter description.

Link: https://lore.kernel.org/all/169556257133.146934.13560704846459957726.stgit@devnote2/

Fixes: cb16330d

 ("fprobe: Pass return address to the handlers")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Florent Revest <revest@chromium.org>