Doc: Fix link to Scrollbar size property

The \l command with just one argument, size, has a competing
section1 link target, Size, in a different C++ file. Specifying the C++
class as the first argument to the link command will make sure that
the link goes to its intended target.

Fixes: QTBUG-103250
Pick-to: 6.3
Change-Id: Ibabd04986e033f7c401f9132d25be7c6f4e8e8d9
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/quicktemplates2/qquickscrollbar.cpp

@@ -81,7 +81,7 @@ QT_BEGIN_NAMESPACE
     \list
     \li \l orientation
     \li \l position
-    \li \l size
+    \li \l {ScrollBar::} {size}
     \li \l active
     \endlist