Tidy up docs re: ImpostorWorkflows in RenderTester API

[zach-klippenstein]

Documentation nit: hopefully most folks won't need to know the impostor workflow details, so maybe you want to put the example above this section. Then this heading can be something more like ## Implementation details: imposter workflows.

If you want to just get this merged, I'm happy to take an editorial pass later.

Originally posted by @rjrjr in #89