Avoid exposing reflection in the package API

In Donovan and Kernighan's p333 (section 12.3 Display, a recursive value printer), it is mentioned that

Where possible, you should avoid exposing reflection in the API of a package. We'll define an unexported function display to do the real work of the recursion, and export Display, a simple wrapper around it that accepts an interface{} parameter.

func Display(name string, x interface{}) {
    fmt.Printf("Display %s (%T):\n", name, x)
    display(name, reflection.ValueOf(x))

And the display function prints different contents depending on the Kind of the input reflection value.

I have two questions

1. Why is it better to not expose the reflection in the package API?
2. Why is using an unexposed display function considered as not exposing reflection in the API? Don't we still call reflection.ValueOf() in Display?

I guess I don't know the definition of "exposing reflection in the package API". Does it just refer to the function arguments or both arguments and content? If it's the former case, then there is no need to define both display and Display. If it's the latter case, why is it better?