issue #475: add doc about GKE node pools in terraform

[modular-magician]

Adds brief explanation about why GKE node pools should be defined separately from the cluster resource
when using terraform.

Related to issue #475. PR #1329 added an example to the terraform docs that indicates that it is recommended to create node pools separately from the cluster definition, but there was no information in the docs about why this is recommended. This PR

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• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
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container: Added note about why node pools should be defined separately from the cluster

Derived from GoogleCloudPlatform/magic-modules#5289