Thank you for the follow up.
Unfortunately, there is no documentation available for this topic, as this is not an existing feature. All UI that the control uses (dialogs and pop-ups) can be replaced by custom user-defined ones. In order to do so, you can download the source of the controls from your account and use a modification of the default dialog as custom. This will be achieved by simply marking your implementation with the [CustomSpellCheckingDialog] attribute.
The ChangeAll functionality can be achieved by slightly modifying the Change functionality to iterate through all occurrences of the current spell-checked word. In this regard, it may be easier to pass the RichTextBox being spell-checked to the dialog (e.g. as a property), so that you can use the document, its positions, etc. We will consider adding this functionality on our end in the near future. When it comes to the Ignore functionality, we are not sure what will be the best way to implement it, that is why we cannot offer any suggestions in this regard.
All the best,
the Telerik team