QtNetwork: Remove documentation references to QUdpDatagram.

Fixes  documentation warnings:
qtbase/src/network/kernel/qnetworkinterface.cpp:528: warning: Can't link to 'QUdpDatagram::interfaceIndex()'
qtbase/src/network/kernel/qnetworkinterface.cpp:587: warning: Can't link to 'QUdpDatagram::interfaceIndex()'

Change-Id: I6579f7880b5e183b9c68dfe08fa7671f1511fdfa
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Richard J. Moore <rich@kde.org>

src/network/kernel/qnetworkinterface.cpp

@@ -536,7 +536,7 @@ QList<QNetworkAddressEntry> QNetworkInterface::addressEntries() const
         QNetworkInterface::interfaceFromName(name).index()
     \endcode
 
-    \sa interfaceFromName(), interfaceNameFromIndex(), QUdpDatagram::interfaceIndex()
+    \sa interfaceFromName(), interfaceNameFromIndex()
 */
 int QNetworkInterface::interfaceIndexFromName(const QString &name)
 {
@@ -596,7 +596,7 @@ QNetworkInterface QNetworkInterface::interfaceFromIndex(int index)
         QNetworkInterface::interfaceFromIndex(index).name()
     \endcode
 
-    \sa interfaceFromIndex(), interfaceIndexFromName(), QUdpDatagram::interfaceIndex()
+    \sa interfaceFromIndex(), interfaceIndexFromName()
 */
 QString QNetworkInterface::interfaceNameFromIndex(int index)
 {

src/network/socket/qnativesocketengine.cpp

@@ -111,7 +111,7 @@
     \value WantAll              this is a catch-all value to indicate the caller is
                                 interested in all the available information
 
-    \sa readDatagram(), QUdpDatagram
+    \sa readDatagram()
 */
 
 #include "qnativesocketengine_p.h"