I am afraid this is yet not implemented. We have it logged in our public feedback portal, please cast your vote and follow the item to get notified on status changes at the link below:
In the meantime, you can customize the tooltips through custom renderers on each platform, please take a look at the following kb article for detailed instruction on the approach:
Should you have any additional questions on this, let me know.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items