Retention properties should be documented

It is necessary to update the retention technical documentation and add all the schemas and properties related to the retention. As an example, it is too hard to find the right XPATH from the document export with these properties:

  "isRecord": true,
  "retainUntil": "2021-03-25T14:38:40.308+01:00",
  "hasLegalHold": false,
  "isUnderRetentionOrLegalHold": true,

I guess all the information should be available from https://github.com/nuxeo/nuxeo/tree/master/addons/nuxeo-retention/nuxeo-retention-core/src/main/resources/OSGI-INF