Document that .(un)throttle() are serialized with each other, and with
termios modification by the tty layer.
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>