Generate wxEVT_CONTEXT_MENU everywhere in generic wxTreeCtrl

Clicking outside of the items area didn't generate wxEVT_CONTEXT_MENU in
the generic version, unlike the native MSW one and contrary to
expectations.

Also update the documentation to make it clear when exactly are
wxEVT_TREE_ITEM_MENU and wxEVT_CONTEXT_MENU events generated.

Closes #17361.

src/generic/treectlg.cpp

@@ -3743,7 +3743,11 @@ void wxGenericTreeCtrl::OnMouse( wxMouseEvent &event )
 
         m_dragCount = 0;
 
-        if (item == NULL) return;  /* we hit the blank area */
+        if ( item == NULL )
+        {
+            event.Skip();
+            return;
+        }
 
         if ( event.RightDown() )
         {