Fix missing docs and consistency issues with $page_top and $page_bottom

While writing docs for the introduction of $page_top and $page_bottom, I've stumbled on issues with how the documentation was not updated in system module's page.tpl.php, and how $page_top is not consistent with its wrapper markup to $page_bottom (which lacks any wrapper markup). We should either have suggested / default wrapper markup for both of these or none of these. The toolbar works either way.