[css-grid] Clarifications to z-index. <http://lists.w3.org/Archives/Public/www-style/2014Feb/0288.html>
[css-grid] Slightly reword the definition of 'available space'.

--HG--
extra : rebase_source : 3055bb4a7d43f6e0bca39e715aae9f064bd3b56b