Tooltips in indexTerms.html

[MarcelM]

Every indexterm has a list of linked destination addresses. These links are displayed as superscripted numbers.
These numbers do not help the reader. So I want to give them a tooltip containing the title of the linked topic.
For this purpose the a-link needs a title-attribute.

How do I have to change the transformation so that this title attribute is generated and filled with the correct content?