[css-grid] Expand grid's restriction on ::first-line/letter to match Flexbox.

Author: tabatkins

css-grid/Overview.bs

@@ -563,7 +563,8 @@ Establishing Grid Containers: the ''display/grid'' and ''inline-grid'' 'display'
 			'vertical-align' has no effect on a grid item.
 
 		<li>
-			the ''::first-line'' and ''::first-letter'' pseudo-elements do not apply to <a>grid containers</a>.
+			the ''::first-line'' and ''::first-letter'' pseudo-elements do not apply to <a>grid containers</a>,
+			and <a>grid containers</a> do not contribute a first formatted line or first letter to their ancestors.
 	</ul>
 
 	If an element's specified 'display' is ''inline-grid''

css-grid/Overview.html

@@ -1336,7 +1336,8 @@ <h3 class="heading settled" data-level=3.1 id=grid-containers><span class=secno>
 			<a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visudet.html#propdef-vertical-align>vertical-align</a> has no effect on a grid item.
 
 		<li>
-			the <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-pseudo-4/#selectordef-first-line>::first-line</a> and <span class=css>::first-letter</span> pseudo-elements do not apply to <a data-link-type=dfn href=#grid-container>grid containers</a>.
+			the <a class=css data-link-type=maybe href=http://dev.w3.org/csswg/css-pseudo-4/#selectordef-first-line>::first-line</a> and <span class=css>::first-letter</span> pseudo-elements do not apply to <a data-link-type=dfn href=#grid-container>grid containers</a>,
+			and <a data-link-type=dfn href=#grid-container>grid containers</a> do not contribute a first formatted line or first letter to their ancestors.
 	</ul>
 
 <p>If an element’s specified <a class=property data-link-type=propdesc href=http://dev.w3.org/csswg/css2/visuren.html#propdef-display>display</a> is <a class=css data-link-type=maybe href=#valdef-display-inline-grid>inline-grid</a>