Documentation: seccomp: Fix user notification documentation

The documentation had some previously incorrect information about how
userspace notifications (and responses) were handled due to a change
from a previously proposed patchset.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Fixes: 6a21cc50 ("seccomp: add a return code to trap to userspace")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210517193908.3113-2-sargun@sargun.me