[Internal][Docs]: Update SN ITSM prerequisite integration user current doc to add 'rest_service' role for troubleshooting

### Description

Our [current doc](https://www.elastic.co/guide/en/kibana/8.17/servicenow-action-type.html#servicenow-itsm-connector-prerequisites-integration-user) for servicenow itsm connector prerequisites integration user lists below four users

- import_set_loader
- import_transformer
- personalize_choices
- x_elas2_inc_int.integration_user

However sometimes troubleshooting an issue becomes hard when user receives `errorResponse was null`. In such cases adding `rest_service ` role could help with troubleshooting the issue.

This role is not required for every environment, so it doesn't belong in the mandatory roles list.
A short “If you receive an empty response or the message ‘errorResponse was null,’ temporarily assign the `rest_service` role to your integration user and re-test” message keeps the main prerequisites clean while giving admins a clear next step when the default roles prove insufficient.

### Resources

https://www.elastic.co/guide/en/kibana/8.17/servicenow-action-type.html#servicenow-itsm-connector-prerequisites-integration-user

### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

all

### What release is this request related to?

N/A

### Serverless release

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @js-jankisalvi

**Stakeholders:** @cnasikas @heespi