What is transaction-managed data blocks?
It's a block that can be modified transactionally.  

The best explanation is probably from page 106 in Tom Kyte's Expert Oracle Database Architecture


  ... transaction information when relevant (only blocks that are
  transaction-managed have this information-- a temporary sort block
  would not, for example) regarding active and past transactions on
  the block ...


Most blocks, of course, are subject to transactional modification so they would have transaction information.