set_pred values pre = argument documentation

[EmilHvitfeldt]

I got around to it after a bit. But it is not entirely clear from a cursory glance how these functions are used and what they modify.

Stating that these functions can only modify and return ``x(our data) might be worth explicitly spelling out. It took me a little while to realize I couldn't modifyobject` in here.

Showing a template function in the documentation might be nice 😄

function(x, object) {
  # some code

  x
}