docs: add allowCustomFetcher config flag documentation for TypeScript SDK

fern/products/sdks/overview/typescript/configuration.mdx

@@ -16,6 +16,40 @@ groups:
           noSerdeLayer: false
 ```
 
+<ParamField path="allowCustomFetcher" type="boolean" default="false" toc={true}>
+When enabled, the generated SDK accepts a `fetcher` parameter in the client options. This allows you to wrap the default fetcher with custom logic, enabling dynamic authentication patterns like JWT signing, OAuth token refresh, or HMAC signing.
+
+The custom fetcher acts as middleware for all HTTP requests, giving you a single injection point for authentication logic without overriding individual methods.
+
+```yaml title="generators.yml"
+config:
+  allowCustomFetcher: true
+```
+
+With this enabled, you can provide a custom fetcher when instantiating the client:
+
+```typescript
+import { PlantStoreClient } from "plant-store-sdk";
+import { fetcher as defaultFetcher } from "plant-store-sdk/core";
+
+const customFetcher = async (args) => {
+  // Add custom authentication logic here
+  const headers = {
+    ...args.headers,
+    Authorization: `Bearer ${getToken()}`,
+  };
+  return defaultFetcher({ ...args, headers });
+};
+
+const client = new PlantStoreClient({
+  environment: "https://api.plantstore.com",
+  fetcher: customFetcher,
+});
+```
+
+See [Dynamic authentication](/sdks/generators/typescript/dynamic-authentication) for detailed implementation examples.
+</ParamField>
+
 <ParamField path="allowExtraFields" type="boolean" toc={true}>
 Allow fields that are not defined in object schemas. This only applies to serde.
 
@@ -555,4 +589,4 @@ interface ObjectWithLongAndBigInt {
 	printMyString(myString);
 	```
 
-</ParamField>
+</ParamField>