ConditionsField claimed support for taking a ConditionChoices class

or a callable returning the class, but not an instance. We have call

sites that passed in a class accordingly. The problem with prioritizing

classes is that there's no ability for code to register choices into a

ConditionChoices after construction (say, for extensions).
In practice, this field supported instances just fine, and we even had

unit tests for it, but we now document this explicitly (fixing class

names in the process).
The field has also been updated with typing, signature matches, and

some code cleanup.

Unit tests pass.