Update UITour documentation

I might have [found something](http://mxr.mozilla.org/mozilla-central/source/toolkit/components/search/nsSearchService.js#2441). If I'm right and we're using the XML filename, there's no rule: mostly lowercase, but we have ```eBay``` even in en-US, and plenty of random names in locales (typical example ```wikipedia-es```, but weird cases like ```be-x-old.wikipedia.org```).