Update audit hook documentation for identity constraints

[jewzaam]

Document that for audit hook you have to specify all the fields with an identity constraint in the hook's projection, else the hook will fail. Should include the message you'd get on such failure, to make it easier to find and fix.

[dcrissman]

What is that error message?

[jewzaam]

Great question. Could take a look at the unit tests and see what happens in that case. Pretty sure that's included as a unit test..