Comment 2 for bug 1401643

https://maas.ubuntu.com/docs1.7/development/preseeds.html does not mention prefix curtin_userdata as the normal prefix for fast installs. (Nor does /usr/lib/python2.7/dist-packages/maasserver/preseed.py, from which the documentation was presumably derived) This needs to be added.

It would also be good if a complete example were worked through in the documentation, explaining when the different prefixes would be used.