[css-grid] Fix missing percentages line in grid-*-gap

Author: fantasai

css-grid/Overview.bs

@@ -3204,7 +3204,7 @@ Gutters: the 'grid-column-gap', 'grid-row-gap', and 'grid-gap' properties</h3>
 	Initial: 0
 	Applies to: <a>grid containers</a>
 	Inherited: no
-	Percentages: n/a
+	Percentages: refer to corresponding dimension of the content area
 	Media: visual
 	Computed value: as specified, with <<length>>s made absolute
 	Animatable: as length, percentage, or calc
@@ -3237,12 +3237,13 @@ Gutters: the 'grid-column-gap', 'grid-row-gap', and 'grid-gap' properties</h3>
 	<pre class='propdef'>
 	Name: grid-gap
 	Value: <<'grid-row-gap'>> <<'grid-column-gap'>>?
-	Initial: see individual properties
-	Applies to: see individual properties
-	Inherited: see individual properties
-	Percentages: see individual properties
-	Media: see individual properties
-	Computed value: see individual properties
+	Initial: 0 0
+	Applies to: <a>grid containers</a>
+	Inherited: no
+	Percentages: refer to corresponding dimension of the content area
+	Media: visual
+	Computed value: as specified, with <<length>>s made absolute
+	Animatable: as length, percentage, or calc
 	</pre>
 
 	This property is a <a>shorthand</a> that sets 'grid-row-gap' and 'grid-column-gap' in one declaration.