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/listbox.h

@@ -174,6 +174,18 @@ public:
 
     virtual void SetSelection(int n);
 
+    /**
+        Returns the index of the selected item or @c wxNOT_FOUND if no item is
+        selected.
+
+        @return The position of the current selection.
+
+        @remarks This method can be used with single selection list boxes only,
+                 you must use wxListBox::GetSelections() for the list
+                 boxes with wxLB_MULTIPLE style.
+
+        @see SetSelection(), GetStringSelection()
+    */
     virtual int GetSelection() const;
 
     virtual bool SetStringSelection(const wxString& s, bool select);