Update "Use Azure Key Vault secrets in customization files"

[MaxHorstmann]

Tweak copy to reflect that only user customizations require additional role assignments now.

Do not merge yet, the feature change is currently under consideration but hasn't been implemented.

[prmerger-automator]

@MaxHorstmann : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 6ba862e:

✅ Validation status: passed

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articles/dev-box/how-to-use-secrets-customization-files.md	✅Succeeded

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[v-dirichards]

@RoseHJM

Can you review the proposed changes?

Note that the contributor is not ready to merge this PR yet.

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[RoseHJM]

@RoseHJM

Can you review the proposed changes?

Note that the contributor is not ready to merge this PR yet.

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team

Approved for when @MaxHorstmann is ready to publish.