Add a documentation note to update the slot after syncing

Add a documentation note to update the slot after syncing

Clients may not care about the events generated during SYN_DROPPED, but the
current slot must be updated to avoid a client being out-of-date.
Same with tracking IDs, if they changed, the caller will likely have to update
some internal states.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>