Add manual/FAQ entry explaining the relationship revealing
Copying @akwizgran's explanation as summary:
When a Private Group is created, messages are only synced via the Creator of the group by default. This is done for two reasons: other members of the group may not be contacts, and thus may not be able to sync with each other; or they may be contacts, but may not wish the rest of the group to know that. If two members of the group carry on a conversation while the creator is offline this reveals that they were syncing the group with each other while the creator was offline, so they must be contacts. Therefore members other than the creator don't sync with each other by default. But if two members want to sync the group between each other and don't mind revealing to the group that they're contacts, they can do this.