Documentation: tracing: Add a note about argument and retval access

Add a note about the argument and return value accecss will be best
effort. Depending on the type, it will be passed via stack or a
pair of the registers, but $argN and $retval only support the
single register access.

Link: https://lore.kernel.org/all/169556269377.146934.14829235476649685954.stgit@devnote2/



Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>