Add details for Views integration: the database user permissions if CiviCRM is in a separate database from Drupal

This section https://docs.civicrm.org/sysadmin/en/latest/integration/drupal/views/#drupal-9 leaves out details that the database user should have access to both databases if CiviCRM is in a separate database from Drupal. If not, then any attempt in Views to JOIN CiviCRM tables to Drupal tables will fail. This can happen if adding a Relationship of the User to CiviCRM contacts.