Improve QML Date object documentation.

Carrying on from https://codereview.qt-project.org/#change,61255, I've
added documentation for the from*String() functions, as they weren't
actually documented. I've also formatted some arguments and linked to
Locale documentation where the format enums are mentioned. Note that,
because of QTBUG-32505, I can't link to the format enum directly, so I
just link to the Locale page itself.

The import QtQuick 2.0 should also be removed from the documentation
when that bug is fixed.

Task-number: QTBUG-32492

Change-Id: I19c6c8450746ef2a68bc55fd3ef7db776f4acc35
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>