docs(exporter): adds the showAllOffsets configuration to example Exporter config

[PaulRMellor]

Documentation

Adds the showAllOffsets property to the example configuration of the Kafka Exporter.
The property is useful for stopping collecting metrics on disconnected consumers, which can skew metrics collection.
Plus some minor rewrites to the section.

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• Write tests
• Make sure all tests pass
• Update documentation
• Check RBAC rights for Kubernetes / OpenShift roles
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• Reference relevant issue(s) and close them after merging
• Update CHANGELOG.md
• Supply screenshots for visual changes, such as Grafana dashboards

[ppatierno]

LGTM.