As far as I know, the official Liferay docs do not explain how Service Builder provides transaction management. The feature is documented in the forums and on other websites, but these docs are not exhaustive, may contain errors, or may be removed at any time, so official documentation from Liferay is needed.
Official documentation should confirm whether it is accurate that service methods beginning with add, update, delete etc. are automatically treated as transactions and completely rolled back if there are any errors. The documentation should explain which prefixes cause Liferay to treat a method as a transaction and which method names would not be treated as a transaction (if any). The documentation might also explain the problem of why transactions are needed to keep data consistent.