Fix rate limiting options values

[modular-magician]

Followup to hashicorp/terraform-provider-google#10020

Seems like some of the values were from the EAP/alpha API, but got renamed going to beta (I can see them in internal protos). I didn't catch it in the initial review, "threshold" and "throttle" looked pretty similar as one of four options.

Pulled the text directly from c.g.c. For the ones w/ a well-formatted entry there I removed the list in the inline Description, but for the one where it isn't I used the raw string.

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Release Note Template for Downstream PRs (will be copied)

compute: corrected an issue in `google_compute_security_policy` where only alpha values for certain enums were accepted

Derived from GoogleCloudPlatform/magic-modules#5691