Illustrate how a Pubsub Subscription ttl would be specified as indefinite

[ethankent]

Because of the way a human might skim this doc, some are likely to wind up with the wrong default value. Imagine that someone has gone straight to the ttl section and missed the reference to a 31 day default in the expiration_policy section. They would likely then create an empty expiration_policy block and get a validation error. After removing that block altogether the validation error would disappear, but they would be left with a 31 day expiration rather than the indefinite one they had desired.

[github-actions]

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[SarahFrench]

Hi @ethankent - could you please open a new PR in the upstream GoogleCloudPlatform/magic-modules repo for this change? I believe you'd need to update this line to cause the documentation to be updated like in this PR. Thanks!

[github-actions]

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