GtkNotebook: Clarify a switch statement

We were intentionally falling through here, this commit
just adds comments to make that explicit.

gtk/gtknotebook.c

@@ -2495,6 +2495,7 @@ gtk_notebook_size_allocate (GtkWidget     *widget,
                 {
                 case GTK_POS_TOP:
                   child_allocation.y += priv->cur_page->requisition.height;
+                  /* fall thru */
                 case GTK_POS_BOTTOM:
                   child_allocation.height =
                     MAX (1, child_allocation.height -
@@ -2502,6 +2503,7 @@ gtk_notebook_size_allocate (GtkWidget     *widget,
                   break;
                 case GTK_POS_LEFT:
                   child_allocation.x += priv->cur_page->requisition.width;
+                  /* fall thru */
                 case GTK_POS_RIGHT:
                   child_allocation.width =
                     MAX (1, child_allocation.width -