memorystore: document TOKEN_AUTH as a google-beta-only authorization_mode value

[abdalazizmoh]

Fixes hashicorp/terraform-provider-google#27309.

authorization_mode on google_memorystore_instance has no client-side enum validation, so this is a documentation-only change to the field's description.

I verified TOKEN_AUTH is currently only accepted by the beta (v1beta) API surface, not GA (v1):

• gcloud memorystore instances create --authorization-mode=token-auth (GA track) is rejected client-side: Invalid choice: 'token-auth'. Valid choices are [auth-disabled, iam-auth].
• gcloud beta memorystore instances create --authorization-mode=token-auth (beta track) passes both client-side and server-side validation — a real create request against a test project got all the way to a running operation and only failed on an unrelated (intentionally invalid) network field.

So the description now notes TOKEN_AUTH is only available via the google-beta provider, matching the existing convention for beta-only capabilities documented on otherwise-GA fields (e.g. compute/BackendService.yaml's "Currently self-managed internal load balancing is only available in beta.").

memorystore: documented `TOKEN_AUTH` as a `google-beta`-only value for `authorization_mode` field on `google_memorystore_instance` resource

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@melinath, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.