Add doc for BCDB and DBSession interface

[cendhu]

// BCDB Blockchain Database interface, defines set of APIs
// required to operate with BCDB instance
type BCDB interface {
	// Session instantiates session to the database
	Session(config *config.SessionConfig) (DBSession, error)
}

// DBSession captures user's session
type DBSession interface {
	UsersTx() (UsersTxContext, error)
	DataTx() (DataTxContext, error)
	LoadDataTx(*types.DataTxEnvelope) (LoadedDataTxContext, error)
	DBsTx() (DBsTxContext, error)
	ConfigTx() (ConfigTxContext, error)
	Provenance() (Provenance, error)
	Ledger() (Ledger, error)
}