Update the documentation of QUuid::operator QString

2c8e030a3f forgot to make it obsolete

Change-Id: Ic4d1f04c8c9c6656c3e93c60e7483b14f4517f19
Reviewed-on: http://codereview.qt.nokia.com/1742
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Olivier Goffart <olivier.goffart@nokia.com>

src/corelib/plugin/quuid.cpp

@@ -441,6 +441,7 @@ QUuid QUuid::fromRfc4122(const QByteArray &bytes)
 #ifndef QT_NO_QUUID_STRING
 /*!
     \fn QUuid::operator QString() const
+    \obsolete
 
     Returns the string representation of the uuid.