Make proposal to allow private interfaces in public APIs

[jonmeow]

#931 had an alternative, "Private interfaces in public API files". The question to leads #971 asked if this should be allowed. Per https://github.com/carbon-language/carbon-lang/issues/971#issuecomment-1093636630 the decision is to allow it.

This changes the decision from an earlier proposal (#931), but doesn't have a proposal itself. A standard proposal is still needed for evolution review, essentially translating the issue into a proposed solution with rationale, alternatives considered, etc. This issue is to track the proposal need.

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