[css-grid] Clarify that grid position/span is two numbers, one for each axis. Fixes #1994.

Author: tabatkins

css-grid-1/Overview.bs

@@ -2411,16 +2411,16 @@ Placing Grid Items</h2>
 	<dl export>
 		<dt><dfn export local-lt="position|row position|column position">grid position</dfn>
 		<dd>
-			The <a>grid item</a>’s location in the <a>grid</a>.
+			The <a>grid item</a>’s location in the <a>grid</a> in each axis.
 			A <a>grid position</a> can be either
 			<dfn export lt="definite grid position|definite row position|definite column position|definite position">definite</dfn> (explicitly specified)
 			or <dfn export lt="automatic grid position|automatic row position|automatic column position|automatic position">automatic</dfn> (determined by <a>auto-placement</a>).
 
 		<dt><dfn export local-lt="span|row span|column span">grid span</dfn>
 		<dd>
-			How many <a>grid tracks</a> the <a>grid item</a> occupies.
+			How many <a>grid tracks</a> the <a>grid item</a> occupies in each axis.
 			A <a>grid item</a>’s <a>grid span</a> is always <dfn export lt="definite grid span|definite row span|definite column span|definite span">definite</dfn>,
-			defaulting to 1 if it can't be otherwise determined.
+			defaulting to 1 in each axis if it can't be otherwise determined for that axis.
 	</dl>
 
 	The <dfn export lt="grid-placement property">grid-placement properties</dfn>--