Mention AUTHENTICATED_APP error in docs (saleor#1400)

docs/api-usage/authentication.mdx

@@ -693,6 +693,10 @@ POST /graphql/ HTTP/1.1
 Authorization: Bearer <app-token>
 ```
 
+When calling graphQL, you may see `AUTHENTICATED_APP` error - if this happens, ensure you have attached valid token to the request header.
+
+Apart of the authentication, app must also authorize with valid [permissions](../developer/permissions#app-permissions), that are applied once app is created.
+
 ### Dashboard extensions
 
 Saleor Apps extending the dashboard will be passed a special JSON Web Token (JWT) with the effective permission set equal to the intersection of user and application permissions. This token allows your frontend part to access the API on behalf of the user.