Update documentation of QTextCodec::codecForUtfText

There is no information in the documentation which QTextCodec::codecForUtfText. It can in fact detect UTF16 and UTF32 in LE and BE and also UTF8.

With the history and wrong documentation in QTextStream::*autoDetectUnicode not explicitly saying that it can detect UTF8, it is a bit confusing and without looking at the source impossible to know.