The ARIA support we introduced in RadComboBox is only initial, i.e. we have implemented only base attributes and the control is still not completely screen reader accessible. What I mean is that in addition to the EanbleAriaSupport property you need to set AllowCustomText to true. All this is the reason we have not provided detailed documentation on the subject.
Having all of the above in mind, we are planning a review of the current functionality and will it fully for the next release at the latest (probably it will be available in an internal build before that).
I hope this helps.
All the best,
the Telerik team