Fix problems with GetSelection() documentation

Move wxListBox-specific part to wxListBox documentation, it doesn't
apply to the other controls, such as wxRadioBox.

Also improve the documentation of wxRadioBox::GetSelection() to mention
that it always returns a valid value.

Closes #18968.

interface/wx/radiobox.h

@@ -317,6 +317,13 @@ public:
     virtual unsigned int GetCount() const;
     virtual wxString GetString(unsigned int n) const;
     virtual void SetString(unsigned int n, const wxString& string);
+
+    /**
+        Returns the index of the selected item.
+
+        As radio boxes always have a selected item, the return value is never
+        @c wxNOT_FOUND for this class.
+     */
     virtual int GetSelection() const;
 
 };