Update builtin-library.md

Add explanation on how to configure the documentSelector when using builtin libraries

hugo/content/docs/recipes/builtin-library.md

@@ -163,3 +163,18 @@ export function activate(context: vscode.ExtensionContext) {
 
 This registers an in-memory file system for vscode to use for the `builtin` file schema.
 Every time vscode is supposed to open a file with this schema, it will invoke the `stat` and `readFile` methods of the registered file system provider.
+
+To ensure that LSP services (such as hover, outline, go to definition, etc.) work properly inside a built-in file, make sure that LanguageClientOptions is correctly configured. The document selector used for your language should handle the `builtin` scheme. It is recommended to support all schemes, either by removing the scheme option or by setting the scheme option to '*'.
+```ts
+// Options to control the language client
+clientOptions: LanguageClientOptions = {
+    documentSelector: [{ language: 'mydsl' }],
+}
+```
+or
+```ts
+// Options to control the language client
+clientOptions: LanguageClientOptions = {
+    documentSelector: [{ scheme: '*', language: 'mydsl' }],
+}
+```