Godoc missing from Subscription API

[adambkaplan]

The Subscription API (v1alpha1) is missing godoc descriptions for SubscriptionSpec fields. Godoc descriptions for APIs can help clarify the behavior of OLM subscriptions.

The startingCSV field in particular needs a descriptive comment. For example, can this be used to "pin" the version of a deployed operator if such version exists in the referenced catalog?

See https://github.com/operator-framework/api/blob/master/pkg/operators/v1alpha1/subscription_types.go#L30-L39

[github-actions]

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