Document that PPC functions are also for prior PCs

[jgabry]

Just noticed that this isn’t discussed anywhere, but the PPC functions have always also been very useful for prior predictive checking, not just posterior checking.

[jgabry]

I added a few mentions of this to the doc in 8352a81, but I'm going to leave this issue open because I want to eventually add a vignette (or section of a vignette) about this.

[jgabry]

And prior PCs will be more flexible once we have #151. Currently y needs to be included, which is not always desirable.