Document what constitutes public API (#2175)

Motivation:

SemVer packages should descibe what constitutes their API.

Modifications:

- Add a doc explaining what the public API is

Result:

More docs

Sources/GRPCCore/Documentation.docc/Articles/Public-API.md

@@ -0,0 +1,93 @@
+# Public API
+
+Understand what constitutes the public API of gRPC Swift and the commitments
+made by the maintainers.
+
+## Overview
+
+The gRPC Swift project uses [Semantic Versioning 2.0.0][0] which requires
+projects to declare their public API. This document describes what is and isn't
+part of the public API; commitments the maintainers make relating to the API;
+and guidelines for users.
+
+For clarity the project is comprised of the following Swift packages:
+
+- [grpc/grpc-swift][1],
+- [grpc/grpc-swift-nio-transport][2],
+- [grpc/grpc-swift-protobuf][3], and
+- [grpc/grpc-swift-extras][4].
+
+## What _is_ and _isn't_ public API
+
+### Library targets
+
+All library targets made available as package products as considered to be
+public API. Examples of these include `GRPCCore` and `GRPCProtobuf`.
+
+> Exceptions:
+> Targets with names starting with an underscore (`_`) aren't public API.
+
+### Symbols
+
+All publicly exposed symbols (i.e. symbols which are declared as `public`)
+within public library targets or those which are re-exported from non-public
+targets are part of the public API. Examples include `Metadata`,
+`ServiceConfig`, and `GRPCServer`.
+
+> Exceptions:
+> - Symbols starting with an underscore (`_`), for example `_someFunction()` and
+>   `_AnotherType` aren't public API.
+> - Initializers where the first character of the first parameter label is an
+>   underscore, for example `init(_foo:)` aren't public API.
+
+### Configuration and inputs
+
+Any configuration, input, and interfaces to executable products which have
+inputs (such as command line arguments, or configuration files) are considered
+to be public API. Examples of these include the configuration file passed to the
+Swift Package Manager build plugin for generating stubs provided by
+[grpc-swift-protobuf][3].
+
+> Exceptions:
+> - Executable _targets_ which aren't exposed as executable _products_.
+
+## Commitments made by the maintainers
+
+Without releasing a new major version, the gRPC Swift maintainers commit to not
+adding any new types to the global namespace without a "GRPC" prefix.
+
+To illustrate this, the maintainers may:
+1. Add a new type to an existing module called `GRPCPanCakes` but will not add a
+   new type called `PanCakes` to an existing module.
+2. Add a new top-level function to an existing module called `grpcRun()` but
+   won't add a new top-level function called `run()`.
+3. Add a new module called `GRPCFoo`. Any symbols added to the new module at the
+   point the module becomes API aren't required to have a "GRPC" prefix; symbols
+   added after that point will be prefixed as required by (1) and (2).
+
+This allows the project to follow Semantic versioning without breaking adopter
+code in minor and patch releases.
+
+## Guidelines for users
+
+In order to not have your code broken by a gRPC Swift update you should only use
+the public API as described above. There are a number of other guidelines you
+should follow as well:
+
+1. You _may_ conform your own types to protocols provided by gRPC Swift.
+2. You _may_ conform types provided by gRPC Swift to your own protocols.
+3. You _mustn't_ conform types provided by gRPC Swift to protocols that you
+   don't own, and you mustn't conform types you don't own to protocols provided
+   by gRPC Swift.
+4. You _may_ extend types provided by gRPC Swift at `package`, `internal`,
+   `private` or `fileprivate` level.
+5. You _may_ extend types provided by gRPC Swift at `public` access level if
+   doing so means that a symbol clash is impossible (such as including a type
+   you own in the signature, or prefixing the method with the namespace of your
+   package in much the same way that gRPC Swift will prefix new symbols).
+
+[0]: https://semver.org
+[1]: https://github.com/grpc/grpc-swift
+[2]: https://github.com/grpc/grpc-swift-nio-transport
+[3]: https://github.com/grpc/grpc-swift-protobuf
+[4]: https://github.com/grpc/grpc-swift-extras

Sources/GRPCCore/Documentation.docc/Documentation.md

@@ -57,6 +57,7 @@ as tutorials.
 ### Project Information
 
 - <doc:Compatibility>
+- <doc:Public-API>
 
 ### Getting involved