Update TS definition for the query prefetch option

[dhritzkiv]

Update the prefetch option to reflect that it's only ever a boolean, not also a function that returns a boolean.

I was attempting to use it as a function that returns a boolean, but it simply gets interpreted as truthy. It seems the source code only expects a boolean.

[japboy]

...or to fix the implementation to support function evaluation as well is another option, i guess?