Update Docs how to use Custom Claims / ID Token with GraphQL API and Data Store

### Is this related to a new or existing framework?

_No response_

### Is this related to a new or existing API?

GraphQL API

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

With the migration from v5 > v6 I was struggling how to pass the ID token with all my custom claims (I generate using a Pre Token Generation Lambda Trigger). Since I was doing this since v5, I needed to look this up for v6 in the docs but did not find it.

The solution is close but not clear! There is a general section how to [Set custom request headers for GraphQL APIs](https://docs.amplify.aws/react-native/build-a-backend/graphqlapi/connect-to-api/#set-custom-request-headers-for-graphql-apis)

```ts
import { Amplify } from 'aws-amplify';
import config from './amplifyconfiguration.json';

Amplify.configure(config, {
API: {
headers: async () => ({
'My-Custom-Header': 'my value'
})
}
});
```

But this is not enough!

### Describe the solution you'd like

There should be a more precise explanation how to pass the ID token when using a Pre Token Generation Lambda Trigger.

```ts
import { Amplify } from 'aws-amplify';
import { fetchAuthSession } from 'aws-amplify/auth';
import config from './amplifyconfiguration.json';

Amplify.configure(config, {
API: {
GraphQL: {
headers: async () => ({
Authorization: (
await fetchAuthSession()
).tokens?.idToken?.toString() as string,
}),
},
},
});
```

### Describe alternatives you've considered

My approach is working fine, just update the docs 😉

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change