qdoc: Brief attribute was written incorrectly

The brief attribute for a C++ property was being written
after the <property> tag had been closed. This has been
fixed by explicitlty writing the brief attribute for each
XML tag that has a brief text to write.

Task-number: QTBUG-31149
Change-Id: Ic3857b8fa50cfb84536b838771751d8b92ed590e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>