Expand documentation of elastic-operator command-line

[rseldner]

Proposal

The Configure ECK documentation contains references to an elastic-operator command line tool.

ECK can be configured using either command line flags or environment variables.
...
The operator can be started using any of the following methods to achieve the same end result:

Configuration file method
./elastic-operator manager --config=eck-config.yaml
Command-line flags method
./elastic-operator manager --log-verbosity=2 --metrics-port=6060 --namespaces=ns1,ns2,ns3
Environment variables method
LOG_VERBOSITY=2 METRICS_PORT=6060 NAMESPACES="ns1,ns2,ns3" ./elastic-operator manager

Source: https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-operator-config.html

There is no explanation on how to use elastic-operator nor where it resides.

It would be helpful to new users to document instructions on how to actually use these command line methods.