Add QSGPaintedItem docs to module and fix non-standard wording.

Change-Id: I763b1fdabdac36935bd14005a9e38b2494319726
Reviewed-on: http://codereview.qt-project.org/5521


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>