docs: finish public API definition (#567)

The backend is built with the goal of providing an HTTP API. This means that
the go module itself does not need to be part of the public API to guarantee
stable versioning for users.

If other projects do start relying on it as a go module and deem a more stable
approach necessary, their maintainers are welcome to open an issue to discuss
adding the exported functions and methods to the public API definition for this.

For now however, it will not be part of the public API.

Author: morremeyer

README.md

@@ -97,6 +97,9 @@ This project is versioned using [Semantic Versioning 2.0.0](https://semver.org/s
 
 The public API is defined as:
 
-- The API endpoints, meaning the routes, parameters, HTTP respone codes and response body format (for a function, we'd just say “the function signature“)
+- The API endpoints, meaning the routes, parameters, HTTP respone codes and response body format (for a function, we'd just say “the function signature“) that the HTTP server built from the code in this repository provides
 - The behavior of the API endpoints no matter the signature of the endpoint is
-- Planned: ~~Public functions and methods of the code - this enables anybody wrapping the backend (e.g. the standalone version) to rely on the version numbers.~~ This will be added to the definition once all methods and functions intended to be private are private. Track [#423](https://github.com/envelope-zero/backend/issues/423) if you're interested in this.
+
+Not part of the public API is:
+
+- The go module itself, meaning that exported function signatures or behaviors can change at any point in time