In short, -a or --attach means you attach to standard sreams which will happen by default unless passing -d or --detach, so yes to the question 1., but docker start or docker container start by default will not attach to STDIN as it also menitoned in the output of docker start --help
-a, --attach Attach STDOUT/STDERR and forward signals
But it is true, that @meyay gave you a way to check the config even if the behaviour changes in the future. Currently you can also use this command as well to get the the values of the relevant parameters:
docker inspect <container name or id> | grep -A5 AttachStdin
But it may not work in the future if the structure of the config changes.