KFileOperator: fix apidox of fileHighlighted()

Summary:
Since 966f0b2054 we are no longer using pointers.

This signal is emitted with a default-contructed KFileItem in
multiselection mode or without selections (see _k_slotSelectionChanged()).

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12599