PDF: add a comment to what otherwise looks like an unused method

I'm not 100% sure we still support WebKit, still leave a note.

This work has been kindly sponsored by the QGIS project
(https://qgis.org/).

Change-Id: I45d4351fd4a106763f26685a5510ed630d752745
Reviewed-by: Tobias Koenig <tobias.koenig@kdab.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/painting/qpdf.cpp

@@ -1035,6 +1035,7 @@ void QPdfEngine::drawTextItem(const QPointF &p, const QTextItem &textItem)
     *d->currentPage << "Q\n";
 }
 
+// Used by QtWebKit
 void QPdfEngine::drawHyperlink(const QRectF &r, const QUrl &url)
 {
     Q_D(QPdfEngine);