RadCombobox WAI-ARIA improvements 

1. When we have a selection but do not have filtering and custom text:

aria-autocomplete: list ---> Combobox list item
2. When we have selection and filtering: ---> Combobox inline completion list item
aria-autocomplete: both
3. disabled: aria-autocomplete: none 

https://www.w3.org/TR/wai-aria-practices/examples/combobox/aria1.0pattern/combobox-autocomplete-both.html