Add env override to subscription.md

[Nyefan]

See issues #216 and keycloak/keycloak-operator#420

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[timflannagan]

Thanks - I had a quick comment about whether we'd want to entertain documenting any config-related properties as their own, separate example. WDYT?

[timflannagan]

I think my preference would be to have the config-related examples be isolated to their own set of examples, vs. baking it into an existing one. Any thoughts on that approach? I feels like we need to eventually translate https://github.com/operator-framework/operator-lifecycle-manager/blob/master/doc/design/subscription-config.md into the olm-docs repository.