Document the deprecation of setRoleNames

The API was deprecated in qtbase to make the QAbstractItemModel API more
consistent. Hence, it is now being put in the porting guide to simplify the
effort for the Qt users.

Task-number: QTBUG-31684
Change-Id: I9350f7752accd90b8fcc9f2a6d53554e3a1e3dd2
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>