Add information about the usage of user/group prefix

Sometime, a user id/droup id is expected to be explicitly prefixed, like user:john or group:marketing. This is, I believe, to avoid confusion since a group can have the exact same id as a user.

But it is not clear when to use that and how the platform handles it. For example, is the prefix stored when a field is User/Group? If not, does Nuxeo filter and remove it at the time a document is stored? If yes, I assume the cleaning will be done only for fields of type User/Group, etc.