Document that seed files are run in transactions

[cirosantilli]

I was doing something further down that raised, so I spent one hour trying to figure out why the first object was not being created =(

[hugocorbucci]

Not only that but also that large files can take a very long time to show any progress as well as lock the database in the process.
Adding BREAK EVAL helps with seed-fu's memory but not with the pending transaction database memory usage.