About "Transaction ID Wraparound"
Why after the database suffers transaction ID wraparound would
  transactions that were in the past appear to be in the future ?


They don't. The quoted text just explains why postgres needs to use modulo 231 arithmatic (which means transactions can wrap around as long as old transactions are 'frozen' early enough):


  Normal XIDs are compared using modulo-2^31 arithmetic. This means that
  for every normal XID, there are two billion XIDs that are "older" and
  two billion that are "newer"


to be specific:


  old row versions must be reassigned the XID FrozenXID sometime before
  they reach the two-billion-transactions-old mark


or wrapping the XID around would cause things to break. To prevent that, postgres will start to emit warnings, and eventually shut down and refuse to start new transactons if necessary:


  If for some reason autovacuum fails to clear old XIDs from a table,
  the system will begin to emit warning messages like this when the
  database's oldest XIDs reach ten million transactions from the
  wraparound point:

WARNING:  database "mydb" must be vacuumed within 177009986 transactions 
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in "mydb". 

  
  (A manual VACUUM should fix the
  problem, as suggested by the hint; but note that the VACUUM must be
  performed by a superuser, else it will fail to process system catalogs
  and thus not be able to advance the database's datfrozenxid.) If these
  warnings are ignored, the system will shut down and refuse to start
  any new transactions once there are fewer than 1 million transactions
  left until wraparound


In other words "transactions that were in the past appear to be in the future" and "data loss" are entirely theoretical and will not be caused by transaction ID wraparound in practice.