Improve "shaped" sample documentation

Also link to it from the functions shown in this sample.

docs/doxygen/mainpages/samples.h

@@ -690,10 +690,14 @@ utility under macOS).
 
 @section page_samples_shaped Shaped Window Sample
 
-@sampleabout{how to implement a shaped or transparent window\, and a window showing/hiding with effect}
+@sampleabout{Showing unusual, e.g. shaped or semi-transparent windows}
+
+This sample shows windows with non-rectangular shapes and non-opaque windows as
+well as how the windows can be shown with a special effect.
 
 @see wxTopLevelWindow::SetShape(), wxTopLevelWindow::SetTransparent(),
-wxWindow::ShowWithEffect(), wxWindow::HideWithEffect()
+wxWindow::ShowWithEffect(), wxWindow::HideWithEffect(),
+wxWindow::SetBackgroundStyle()
 
 @sampledir{shaped}
 

interface/wx/toplevel.h

@@ -589,6 +589,9 @@ public:
         shown the first time it's called (but it can be called again after
         showing the window too).
 
+        See @ref page_samples_shaped "the shaped sample" for an example of
+        using this function.
+
         @param alpha
             Determines how opaque or transparent the window will be, if the
             platform supports the operation. A value of 0 sets the window to be

interface/wx/window.h

@@ -2395,7 +2395,9 @@ public:
         Under wxGTK and wxOSX, you can use ::wxBG_STYLE_TRANSPARENT to obtain
         full transparency of the window background. Note that wxGTK supports
         this only since GTK 2.12 with a compositing manager enabled, call
-        IsTransparentBackgroundSupported() to check whether this is the case.
+        IsTransparentBackgroundSupported() to check whether this is the case,
+        see the example of doing it in @ref page_samples_shaped "the shaped
+        sample".
 
         Also, in order for @c SetBackgroundStyle(wxBG_STYLE_TRANSPARENT) to
         work, it must be called before Create(). If you're using your own