Add tip about 'required' in sonata_type_collection

Document 'required' option behavior in sonata_type_collection
Pass admin through to autocompleteItems callback:
~ modified HelperController::retrieveAutocompleteItemsAction() to pass $targetAdmin through to callback instead of $datagrid
~ updated documentation of the callback
Merge pull request #2353 from caponica/autocomplete_callback_with_admin_v2

Pass admin through to autocompleteItems callback:
Merge pull request #2351 from eikes/master

Add tip about 'required' in sonata_type_collection