Add documentation for binary_authorization block

[wyardley]

Add documentation on the available values for evaluation_mode in the documentation for the binary_authorization block of the google_container_cluster resource.

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• 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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[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.

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[modular-magician]

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

Diff report:

Terraform GA: Diff ( 1 file changed, 3 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 3 insertions(+), 1 deletion(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))