spec: document range-over-func

[rsc]

Tracking bug for adding range-over-func to spec. See #61405.

[gopherbot]

Change https://go.dev/cl/589397 mentions this issue: spec: document for range loop over functions

[3052]

@griesemer this example is not great

// print hello world
f := func(yield func(string) bool) {
	if yield("hello") {
		yield("world")
	}
}
for word := range f {
	println(word)
}

its not clear when yield would return true or false, looking at the example alone