When using Xflow Tasks, it is possible to configure the presented fields as Read Only or Required at via Administration/Xflow Interface/View Configuration/Detail View.
However, there is no place where to configure the Description field as Read Only.
Since this field comes from the WF Template and normally has instructions on how to proceed, it should not be changed.
CA Service Desk Manager 17.3 RU19 or lower
Fix is provided in the 17.3 RU23 patch