Explicitly mark non-private APIs.

[reitermarkus]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Prerequisite for #16831.

[MikeMcQuaid]

Looks good, thanks @reitermarkus! Merging now to avoid conflicts but feel free to offer post-merge feedback, anyone.