Update combobox docs to use setNewItemProvider (#10901)

documentation/components/components-combobox.asciidoc

 
 === Handling New Items
 
-Adding new items is handled by a [interfacename]#NewItemHandler#, which gets the
+Adding new items is handled by a [interfacename]#NewItemProvider#, which gets the
 item caption string as parameter for the [methodname]#accept(String)# method.
 
 
 
 // Allow adding new items and add
 // handling for new items
-select.setNewItemHandler(inputString -> {
+select.setNewItemProvider(inputString -> {
 
     Planet newPlanet = new Planet(planets.size(), inputString);
     planets.add(newPlanet);
     // Update combobox content
     select.setItems(planets);
 
-    // Remember to set the selection to the new item
-    select.setSelectedItem(newPlanet);
+    return Optional.of(newPlanet);
 });
 ----