Could have been clearer but it is there. Here is the relevant section. In short single entry is basically the Account view and Double entry is the full ledger. (Called double because of the hard requirement that all Entries come in pairs.)

> Ledgers are conceptually a data model, represented by three entities: Accounts, Entries and Transactions.

> Most people think of money in terms of what’s theirs, and Accounts are the representation of that point of view. They are the reason why engineers naturally gravitate towards the “simplicity” of single-entry systems. Accounts are both buckets of value, and a particular point of view of how its value changes over time.

> Entries represent the flow of funds between Accounts. Crucially, they are always an exchange of value. Therefore, they always come in pairs: an Entry represents one leg of the exchange.

> The way we ensure that Entries are paired correctly is with Transactions. Ledgers shouldn’t interact with Entries directly, but through the Transaction entity.