Update cache flush procedure documentation

[jwokaty]

Update the cache flushing procedure to include calling AnnotationHub and ExperimentHub constructors so that clashes won't happen during the build. If R has been reinstalled then AnnotationHub and ExperimentHub should be reinstalled before calling the constructors.

It might be helpful to schedule flushing the cache so that issues might be more predictable. (Currently, I do updates if necessary on the last Wednesday of the month. Maybe we can remove the cache maybe twice during a cycle, around the midpoint.)