add docs on public documentation requirement

docs/public-api-documentation.md

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+## Why is my PR labelled with `workflow/pending-public-api`?
+
+To ensure the public SDKs (including the [Cloudflare Terraform Provider]) are
+only using stable and intentionally exposed endpoints, we require that all API
+functionality is backed by public documentation. This can either be
+api.cloudflare.com or developers.cloudflare.com depending on the delivery medium.
+
+## But, wrangler/cloudflared/other project doesn't require public documentation?
+
+On occasion, Cloudflare teams release functionality or tooling specific to the
+systems they are responsible for. [Wrangler] and [cloudflared] are two prominent
+examples of this. In these situations, the teams may choose to use unstable or
+undocumented endpoints as they are able to maintain both internal and external
+compatibility for these tools should something need to change without notice or
+a deprecation period.
+
+Unfortunately, the SDKs are not in the same position and cannot make the same
+guarantees externally due to being an interface for external integrations; not
+an abstraction of the functionality. By only accepting documented API endpoints
+into the SDKs, we establish an API contract with the service teams that ensures
+consumers have a reliable and consistent experience when using them. Should an
+API contract be broken, or need fixing, the service team will be responsible to
+maintain it in such a time that a deprecation notice is issued and integrations
+have a migration period.
+
+[cloudflare terraform provider]: https://github.com/cloudflare/terraform-provider-cloudflare/
+[wrangler]: https://github.com/cloudflare/wrangler2
+[cloudflared]: https://github.com/cloudflare/cloudflared