Clarify File-based grants are not visible with API calls

The following is true of all versions: The APIs (user, role, role mapping) only work with objects that were defined via the API.

Objects (users, roles, mappings) defined in files need to be managed via the files (or the `users` CLI tool which uses the files).

API exploration calls (and consecutively, Kibana) will not report roles and mappings defined in files.

Example, where role production_user is defined in roles.yml and user "test" is a file realm user created with the users CLI tool.

```
curl -X GET -u test:password http://localhost:9200/_xpack/security/_authenticate
{"username":"test","roles":["production_user"],"full_name":null,"email":null,"metadata":{},"enabled":true}[root@localhost x-pack]#

curl -X GET -u test:password http://localhost:9200/_xpack/security/role/production_user
{}

curl -X GET -u elastic:password http://localhost:9200/_xpack/security/user/test
{}
```

Some places to update docs to reflect this behavior:

https://www.elastic.co/guide/en/elastic-stack-overview/6.6/defining-roles.html#roles-management-file

https://www.elastic.co/guide/en/elastic-stack-overview/6.6/mapping-roles.html#mapping-roles-file

Commencez par les pages de documentation defining-roles et mapping-roles liées dans l’issue, puis comparez leurs descriptions avec les exemples d’API fournis. Précisez que les utilisateurs, rôles et mappages définis dans des fichiers ou via la users CLI ne sont pas signalés par les API de gestion correspondantes, et vérifiez la formulation mise à jour avec les deux exemples.

Rédigé par le modèle d'indexation à partir du texte de l'issue.