`NumImagesCounter_RBV` behavior when `DataSource` is `None`

[ericonr]

#95 fixes the logic in the driver, but there's still a documentation or behavior gap here.

Should we set NumImagesCounter_RBV to the amount of expected images when the acquisition completes (given that this PV doesn't need to correspond directly to the amount of NDArrays), or should we simply document that it won't be updated when DataSource is None?

I'm somewhat in favor of the former, but can be convinced otherwise.