Fix QString::sprintf documentation

QString::sprintf does actually support all length modifiers, including
%lld. The format string is also parsed as UTF-8.

What's worthwile to mention, though, is that %lc and %ls is at odds
with the standard, since wchar_t isn't necessarily 16 bits wide.

Change-Id: I30cd22ec5b42035824dd98e3cdcc79d7adcc953a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>