doc: explicitly doc package.exports is breaking

If package authors don't explicitly include all previously supported
entry points introducing package.exports will be a Semver-Major change.

Add a warning about this behavior and offer two potential solutions
for module authors.

Refs: then/is-promise#20

PR-URL: #33074
Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/esm.md

@@ -183,25 +183,75 @@ versions of Node.js, but its capabilities are limited: it only defines the main
 entry point of the package.
 
 The `"exports"` field provides an alternative to `"main"` where the package
-main entry point can be defined while also encapsulating the package, preventing
-any other entry points besides those defined in `"exports"`. If package entry
-points are defined in both `"main"` and `"exports"`, the latter takes precedence
-in versions of Node.js that support `"exports"`. [Conditional Exports][] can
-also be used within `"exports"` to define different package entry points per
-environment, including whether the package is referenced via `require` or via
-`import`.
+main entry point can be defined while also encapsulating the package,
+**preventing any other entry points besides those defined in `"exports"`**.
+This encapsulation allows module authors to define a public interface for
+their package.
 
 If both `"exports"` and `"main"` are defined, the `"exports"` field takes
-precedence over `"main"`.
+precedence over `"main"`. `"exports"` are not specific to ES modules or
+CommonJS; `"main"` will be overridden by `"exports"` if it exists. As such
+`"main"` cannot be used as a fallback for CommonJS but it can be used as a
+fallback for legacy versions of Node.js that do not support the `"exports"`
+field.
+
+[Conditional Exports][] can be used within `"exports"` to define different
+package entry points per environment, including whether the package is
+referenced via `require` or via `import`. For more information about supporting
+both CommonJS and ES Modules in a single package please consult
+[the dual CommonJS/ES module packages section][].
+
+**Warning**: Introducing the `"exports"` field prevents consumers of a package
+from using any entry points that are not defined, including the `package.json`
+(e.g. `require('your-package/package.json')`. **This will likely be a breaking
+change.**
+
+To make the introduction of `"exports"` non-breaking, ensure that every
+previously supported entry point is exported. It is best to explicitly specify
+entry points so that the package’s public API is well-defined. For example,
+a project that previous exported `main`, `lib`,
+`feature`, and the `package.json` could use the following `package.exports`:
 
-Both `"main"` and `"exports"` entry points are not specific to ES modules or
-CommonJS; `"main"` will be overridden by `"exports"` in a `require` so it is
-not a CommonJS fallback.
+```json
+{
+  "name": "my-mod",
+  "exports": {
+    ".": "./lib/index.js",
+    "./lib": "./lib/index.js",
+    "./lib/index": "./lib/index.js",
+    "./lib/index.js": "./lib/index.js",
+    "./feature": "./feature/index.js",
+    "./feature/index.js": "./feature/index.js",
+    "./package.json": "./package.json"
+  }
+}
+```
+
+Alternatively a project could choose to export entire folders:
+
+```json
+{
+  "name": "my-mod",
+  "exports": {
+    ".": "./lib/index.js",
+    "./lib": "./lib/index.js",
+    "./lib/": "./lib/",
+    "./feature": "./feature/index.js",
+    "./feature/": "./feature/",
+    "./package.json": "./package.json"
+  }
+}
+```
 
-This is important with regard to `require`, since `require` of ES module files
-throws an error in all versions of Node.js. To create a package that works both
-in modern Node.js via `import` and `require` and also legacy Node.js versions,
-see [the dual CommonJS/ES module packages section][].
+As a last resort, package encapsulation can be disabled entirely by creating an
+export for the root of the package `"./": "./"`. This will expose every file in
+the package at the cost of disabling the encapsulation and potential tooling
+benefits this provides. As the ES Module loader in Node.js enforces the use of
+[the full specifier path][], exporting the root rather than being explicit
+about entry is less expressive than either of the prior examples. Not only
+will encapsulation be lost but module consumers will be unable to
+`import feature from 'my-mod/feature'` as they will need to provide the full
+path `import feature from 'my-mod/feature/index.js`.
 
 #### Main Entry Point Export
 
@@ -1750,6 +1800,7 @@ success!
 [dynamic instantiate hook]: #esm_code_dynamicinstantiate_code_hook
 [import an ES or CommonJS module for its side effects only]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Import_a_module_for_its_side_effects_only
 [special scheme]: https://url.spec.whatwg.org/#special-scheme
+[the full specifier path]: #esm_mandatory_file_extensions
 [the official standard format]: https://tc39.github.io/ecma262/#sec-modules
 [the dual CommonJS/ES module packages section]: #esm_dual_commonjs_es_module_packages
 [transpiler loader example]: #esm_transpiler_loader