Improve documentation for google_compute_backend_bucket.cdn_policy

[andyyaldoo]

State that default_ttl, client_ttl, max_ttl should be omitted when cache_mode is set to USE_ORIGIN_HEADERS.

Fixes hashicorp/terraform-provider-google#22496

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compute: Improve documentation for `google_compute_backend_bucket.cdn_policy`

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 2 files changed, 18 insertions(+), 12 deletions(-))
google-beta provider: Diff ( 2 files changed, 18 insertions(+), 12 deletions(-))

[modular-magician]

Tests analytics

Total tests: 1173
Passed tests: 1089
Skipped tests: 84
Affected tests: 0

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• compute

🟢 All tests passed!

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[slevenick]

Thanks!