Clarify the role of io.EOF in Decode

[lukemassa]

I was originally confused by the fact that this code produced an error:

	var test struct{}
	err := yaml.NewDecoder(bytes.NewReader([]byte(""))).Decode(&test)
	fmt.Println(test, err)

whereas this did not:

	var test struct{}
	err := yaml.Unmarshal([]byte(""), &test)
	fmt.Println(test, err)

See: https://go.dev/play/p/tM_jp90fjFx

However, when I looked closer, I realized that this was the intended behavior. Decode() decodes the "next" document until it can't find any more, which it then indicates via io.EOF. If it were to return nil, there'd be no way for the client of Decode() to know there were no more documents.

This change adds to the documentation so this is a bit clearer.

Closes: #805