Adding clarification text to gke_hub_feature files that gke_hub is th…

[modular-magician]

…e same as Fleet API.

Later this year, GKE Hub is being renamed to Fleets. This change clarifies in the gke_hub docs specifying that google_gke_hub is the Fleet API.

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Derived from GoogleCloudPlatform/magic-modules#5189

[github-actions]

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