More info in QLocalSocket::socketDescriptor

Documents the returned type of values depending on the platform.

Change-Id: I0659ae8a0a4c5aafd368a25564eca29973702b1e
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/network/socket/qlocalsocket.cpp

@@ -128,6 +128,20 @@ QT_BEGIN_NAMESPACE
 
     The socket descriptor is not available when QLocalSocket
     is in UnconnectedState.
+    The type of the descriptor depends on the platform:
+
+    \list
+        \li On Windows, the returned value is a
+        \l{https://msdn.microsoft.com/en-us/library/windows/desktop/ms740522(v=vs.85).aspx}
+        {Winsock 2 Socket Handle}.
+
+        \li With WinRT and on INTEGRITY, the returned value is the
+        QTcpSocket socket descriptor and the type is defined by
+        \l{QTcpSocket::socketDescriptor}{socketDescriptor}.
+
+        \li On all other UNIX-like operating systems, the type is
+        a file descriptor representing a socket.
+    \endlist
 
     \sa setSocketDescriptor()
 */