Be clear about getter function results being under management of Pulumi

[squaremo]

Getter functions bring a resource into the state, and as such, they have a cost implication for people on paid plans. Therefore:

• mention that getter functions bring a resource under management of Pulumi, in https://www.pulumi.com/docs/intro/concepts/resources/get/
• add .get resources to the list of things included for the purpose of pricing, under https://www.pulumi.com/pricing/#faq-pricing:

This includes provider resources (e.g., an Amazon S3 bucket), [...]