"Use experimental fitz rendering instead of poppler" really doesn't say what actually will happen - install mupdf and use its rendering stack. I would have called the USE flag "mupdf", everything would be clear. But I think changing the description works well, too. "(Experimental!) Instead of poppler use the fitz rendering API that comes with mupdf." Or similar... (My English might not be the very best ;) Tried to make clear that fitz rendering in qpdfview is experimental, not mupdf itself...)
I've changed it into "Use experimental fitz rendering, provided by <pkg>app-text/mupdf</pkg>, instead of poppler"
