1.9 UDC configuration

Synchronises the documentation and the behaviour of how to disable UDC.

This synchronisation goes both ways. The documentation is updated to described the preferred way of disabling UDC. The UDC configuration code is updated to honour the different ways that the manual has previously described to disable UDC.

The UDC configuration code does not fail when an unexpected value is specified as of this change set, instead it falls back to the default value of having UDC enabled.