doc: clarify behavior of k_poll_signal_raise on error

k_poll_signal_raise() returns an error code to indicate that the raise
was too late to notify an expiring poll.  Make clear that this does not
mean that the signal was lost: a subsequent poll will find it and expire
immediately.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
1 file changed