Document GUrl classes

Jürgen Knödlseder
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doc/source/user_manual/modules/support.rst
@@ -278,3 +278,35 @@ Function                       Description
  
 Media independent information handling
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To handle media independent data access, the ``GUrl`` base class has been 
+implemented that represent a media in an abstract way. The class has
+abstract ``open``, ``read``, ``write`` and ``close`` method to open
+a media, read from it or write to it, and to close the media. 
+
+Support is implemented so far for file and string media, but in the futur,
+direct access to ressources over the internet may become possible.
+A file media is implemented by the ``GUrlFile`` class, while a string
+media is implemented by the ``GUrlString`` class.
+
+An example of a class making using of ``GUrl`` is the ``GXml`` class. Look
+at the following code:
+
+.. code-block:: cpp
+   :linenos:
+
+    GXml xml;
+    xml.append(GXmlElement("dummy"));
+    GUrlFile   file("my_file.xml", "w");
+    GUrlString chain;
+    xml.write(file);
+    xml.write(chain);
+
+Line 1 declares a XML object and in line 2 we append a dummy element
+to it. In line 3 we now create a file named ``my_file.xml`` for which
+we allow write access. In line 4 we allocate a string media. We then
+write the XML object first into the file in line 5, and the in the string
+in line 6. This illustrates how the ``GUrl`` classes can be used to
+redirect the same information to different media. Reading from different
+media is analoguous.
+