[Go] Updated the 'callback' and 'extend' examples to match the 'director' one.

After the documentation update on how to utilize the director feature with
commit @17b1c1c the 'callback' and 'extend' examples needed an update as well.

Examples/go/callback/index.html

@@ -12,67 +12,17 @@
 <H2>Implementing C++ callbacks in Go</H2>
 
 <p>
-This example illustrates how to use directors to implement C++
-callbacks in Go.
-</p>
-
-<p>
-Because Go and C++ use inheritance differently, you must call a
-different function to create a class which uses callbacks.  Instead of
-calling the usual constructor function whose name is <tt>New</tt>
-followed by the capitalized name of the class, you call a function
-named <tt>NewDirector</tt> followed by the capitalized name of the
-class.
-</p>
-
-<p>
-The first argument to the <tt>NewDirector</tt> function is an instance
-of a type.  The <tt>NewDirector</tt> function will return an interface
-value as usual.  However, when calling any method on the returned
-value, the program will first check whether the value passed
-to <tt>NewDirector</tt> implements that method.  If it does, the
-method will be called in Go.  This is true whether the method is
-called from Go code or C++ code.
-</p>
-
-<p>
-Note that the Go code will be called with just the Go value, not the
-C++ value.  If the Go code needs to call a C++ method on itself, you
-need to get a copy of the C++ object.  This is typically done as
-follows:
-
-<blockquote>
-<pre>
-type Child struct { abi Parent }
-func (p *Child) ChildMethod() {
-	p.abi.ParentMethod()
-}
-func f() {
-	p := &Child{nil}
-	d := NewDirectorParent(p)
-	p.abi = d
-	...
-}
-</pre>
-</blockquote>
-
-In other words, we first create the Go value.  We pass that to
-the <tt>NewDirector</tt> function to create the C++ value; this C++
-value will be created with an association to the Go value.  We then
-store the C++ value in the Go value, giving us the reverse
-association.  That permits us to call parent methods from the child.
-
-</p>
-
-<p>
-To delete a director object, use the function <tt>DeleteDirector</tt>
-followed by the capitalized name of the class.
+This example illustrates how to use directors to implement C++ callbacks in Go.
+See the <a href="../../../Doc/Manual/Go.html#Go_director_classes">Go Director
+Classes</a> documentation subsection for an in-depth explanation how to use the
+director feature.
 </p>
 
 <p>
 <ul>
-<li><a href="example.h">example.h</a>.  Header file containing some enums.
-<li><a href="example.i">example.i</a>.  Interface file.
+<li><a href="example.h">example.h</a>.  Header with the definition of the Caller and Callback classes.
+<li><a href="example.i">example.i</a>.  SWIG interface file.
+<li><a href="gocallback.go">gocallback.go</a>.  Go source with the definition of the GoCallback class.
 <li><a href="runme.go">runme.go</a>. Sample Go program.
 </ul>