Improve QCollator documentation

QCollator documentation right now is pretty bare and not well interlinked:

• There's no simple snippet showing how to use QCollator for sorting strings (e.g. with std::sort):

  QCollator collator;
  std::sort(list.begin(), list.end(), collator);
  

• There's no simple snippet for doing the same using QCollator::sortKey(), i.e.
• QStringList::sort documentation does not reference QCollator
• QString::localeAwareCompare documentation does not reference QCollator.