docs: prefer ImportStateIdFunc helpers from acctest package

[jar-b]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

Replace custom import state ID function examples with the acctest.AttrImportStateIdFunc and acctest.AttrsImportStateIdFunc functions as the preferred best practice.

[github-actions]

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[ewbankkit]

LGTM 🚀.

[github-actions]

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