Document CLI env flags override TestSuite file

The kuttl cli overrides the testsuite file... with the exception of the test env. (this is a require to fix that).

If the `kuttl-test.yaml` has a configuration for `startControlPlane: true` and the cli is started with `--start-kind`.... the cli override should start kind.

Perhaps we should have an override that allows to point at a cluster (currently it is the default that is selected if nothing is provided).

Start by tracing how the kuttl CLI reads kuttl-test.yaml and handles the --start-kind flag, then compare those values with startControlPlane: true. Clarify whether the requested cluster-target override is part of the same change; done means CLI environment flags take precedence over the TestSuite configuration and the behavior is covered by the relevant CLI tests.

Written by the indexing model from the issue text.