Dock widgets enable the QTabBar::drawBase property, but the clip region
left a gap of 3 pixels on each side between the base line and the tabs.
Correct the size of the hole cut into the clip region accordingly.
Visually tested on macOS 10.15.7 and macOS 11.2.1 using the dockwidgets
and tabdialog example.
As a drive-by, fix a spelling mistake in related documentation.
Fixes: QTBUG-86362
Pick-to: 6.2 6.1
Change-Id: Iada027999467741d0590576ee4508e6f41fb056d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
f5d6f8dca0