docs: describe the config file format in the CLI docs

[ringerc]

What this PR does / why we need it:

Update the CLI documentation to provide some information on how to use the config file for more complex kube-state-metrics, instead of complex and very long command lines.

How does this change affect the cardinality of KSM:

No change. Documentation only.

[linux-foundation-easycla]

• ❌ - login: @ringerc / name: Craig Ringer . The commit (45fe400) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[k8s-ci-robot]

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[k8s-ci-robot]

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[logicalhan]

/assign @rexagod
/triage accepted

[ringerc]

Yes I know I need to sign the CLA. My org is being painfully slow about assigning a CLA manager.