Update documentation for resources that don't support gcloud's syntax

[karolgorc]

References issue #14260

Changing the documentation for these resources to be at line with how the code actually works.

• compute_instance_template
• compute_region_instance_template
• compute_snapshot

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[modular-magician]

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Diff report

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

google provider: Diff ( 3 files changed, 3 insertions(+), 3 deletions(-))
google-beta provider: Diff ( 3 files changed, 3 insertions(+), 3 deletions(-))

[shuyama1]

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