Clarify example at dockerhub

Now at https://hub.docker.com/_/influxdb (Database Initialization: Manually Initializing the Database):
```
$ docker run --rm \
-e INFLUXDB_DB=db0 -e INFLUXDB_ADMIN_ENABLED=true \
-e INFLUXDB_ADMIN_USER=admin -e INFLUXDB_ADMIN_PASSWORD=supersecretpassword \
-e INFLUXDB_USER=telegraf -e INFLUXDB_USER_PASSWORD=secretpassword \
-v $PWD:/var/lib/influxdb \
influxdb /init-influxdb.sh
```
But without INFLUXDB_HTTP_AUTH_ENABLED=true all this credentials is useless, isn't it?
I got a little bit confused.

Start with the Docker Hub InfluxDB page's “Database Initialization: Manually Initializing the Database” example and review the documented environment variables, especially INFLUXDB_HTTP_AUTH_ENABLED. Clarify whether the credentials are effective without that setting, and update the example or explanation so the authentication behavior is unambiguous.

Written by the indexing model from the issue text.