docs: document package.json's "exports" fields' conditional exports

docs/lib/content/configuring-npm/package-json.md

@@ -341,7 +341,7 @@ If `main` is not set, it defaults to `index.js` in the package's root folder.
 
 ### exports
 
-The exports field is an object that maps entry points to modules. It supports wild cards (`*`) and explicit names.
+The exports field is an object that maps entry points to modules. This field is supported by Node.js versions including and higher than 12. It acts a more featureful alternative to the `main` field. It supports wild cards (`*`) and explicit names.
 
 For example, you could have:
 
@@ -351,14 +351,16 @@ For example, you could have:
  ".": "./index.js",
  "./*": "./*.js",
  "./*.js": "./*.js",
- "./foo": "./path/to/foo.js"
+ "./foo": "./path/to/foo.js",
+ "./package.json": "./package.json",
+ "./
  }
 }
 ```
 
 If someone installed your package with this in your `package.json`, they could `require("my-package")` and it would be mapped to `./node_modules/my-package/index.js` because of `".": "./index.js"`.<br>
 If they did `require("my-package/bar")` or `require("my-package/bar.js")`, it would be mapped to `./node_modules/my-package/bar.js` because of the wild cards (`*`).<br>
-If they did `require("my-package/foo")` it would be mapped to `./node_modules/my-package/path/to/foo.js` because of the explicit mapping `"./foo": "./path/to/foo.js"`.<br>
+If they did `require("my-package/foo")` it would be mapped to `./node_modules/my-package/path/to/foo.js` because of the explicit mapping `"./foo": "./path/to/foo.js"`.
 
 ### browser