docs: New IAB "prorates" feature

Added description of new prorated seasonal subscription prices. Also
noted that free trials are now supported for seasonal subscriptions.

Per Z's request, also folded in a bugfix that had been waiting a
while.

See first comment for doc stage location.

bug: 25756808
bug: 14070005
Change-Id: Id43ed7bd6f84ec0ba9a05c4202fc2585c7fd6a22