Adding clarification text to gke_hub_feature files that gke_hub is th…

[architaro]

…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.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• REPLACEME

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@ScottSuarez, please review this PR or find an appropriate assignee.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 2 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 3 files changed, 2 insertions(+), 6 deletions(-))

[ScottSuarez]

Do you have corresponding public documentation to reference for this migration/name transition?

[architaro]

Hey Scott - there isn't direct public documentation for this change, but for registering for a cluster we use the name fleets now (https://cloud.google.com/anthos/multicluster-management/connect/registering-a-cluster). Also mentioned here is the Fleet API - https://cloud.google.com/anthos/multicluster-management/connect/prerequisites#enable_apis