fix doc: user token grant response

- clarify that this field is always null (instead of
explaining a lot about the various formats of the access token
when no access token will be returned)

Co-authored-by: Peter Chen <peter-h.chen@broadcom.com>

uaa/src/test/java/org/cloudfoundry/identity/uaa/login/TokenEndpointDocs.java

@@ -382,7 +382,7 @@ void getTokenUsingUserTokenGrant() throws Exception {
         );
 
         Snippet responseFields = responseFields(
-                accessTokenFieldDescriptor,
+                fieldWithPath("access_token").description("This field is always `null`."),
                 tokenTypeFieldDescriptor,
                 expiresInFieldDescriptor,
                 scopeFieldDescriptorWhenUserToken,