QLocale constructor docs don't accurately describe the fall-back process

The documentation of QLocale::QLocale(Language, Script, Country) and the similar without script give a partial account of how we select an actual local based on what was asked for, but it isn't a faithful account of the actual process – which I have made compatible with CLDR's rules in recent work. The present description reflects neither what was true before that work, nor what's now true.
The docs thus need an update to reflect reality.