PDF's Link annotation has no Alternative Description; no "rollover" feature.
That's the case for PDF version 1.2 (Acrobat 3.x product family from back in the '90s) through "PDF-1" (ISO 32000-1:2008, the current ISO Standard for PDF).
TORU supports providing Accessible PDF (PDF that is compliant with ISO 14289-1, PDF/UA-1). TORU doesn't "work" on a feature/attribute of the PDF Link annotation that does not exist.
When a PDF is a well-formed Tagged PDF (ISO 144289-1 compliant) Link annotations will be appropriately tagged with the LINK element. Within the object properties of the LINK element an Alternate Description can be provided. The proper usage of Alternate Description is introduced in ISO 32000-1 and further discussed in ISO 14289-1. Alternate Description in a well-formed Tagged is consumed / used by AT. It does not provide an HTMLish functionality.
Be well...