tty: Document defunct ASYNC_* bits in uapi header

Note the serial_struct flags for which the kernel ignores and performs
no action. The flags cannot be removed since they form part of the
userspace interface via the TIOCSSERIAL/TIOCGSERIAL ioctls.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>