Here is the current bug for the problem:
https://bugs.freenas.org/issues/9148
The easiest solution is to do a manual upgrade (what we use to do before 9.3) and download the latest .tgz Upgrade file (don't forget the sumcheck value) and do the upgrade. A few minutes later you will have the user manual restored.
EDIT: The OP already knows this answer, this is really for anyone else looking for this answer.