PortFast != Spanning tree, don’t mistake.
PortFast is a manner in which you can tell Cisco devices that the port is going to be used by end devices, The port wil therefore go in to a forwarding state without completing extensive tests regarding spanning-tree loops. It will test for these and block the port if a loop is found but this could be a little too late.
The reason i suppose for ConfGen to point this Cisco option out is that it is known to put a port in a forwarding state too slow, therefore missing the initial connection from the client to the LAN (usually the DHCP proces) and leaving the client booting slow and missing stuff. (PXE booting is near impossible without it)
So, a lot of blabla, but in essence, if someone tells you to enable or disable Portfast, dont mess with your Spanning-Tree, its not the same thing.