[css-align] Dfn the older grid-*-gap props.

Author: tabatkins

css-align/Overview.bs

@@ -1732,9 +1732,9 @@ Legacy Gap Properties: the 'grid-row-gap', 'grid-column-gap', and 'grid-gap' pro
 	For legacy compatibility,
 	those legacy property names must be supported as aliases:
 
-	* 'grid-row-gap' must be treated as a shorthand for the 'row-gap' property
-	* 'grid-column-gap' must be treated as a shorthand for the 'column-gap' property
-	* 'grid-gap' must be treated as a shorthand for the 'gap' property
+	* <dfn property>grid-row-gap</dfn> must be treated as a shorthand for the 'row-gap' property
+	* <dfn property>grid-column-gap</dfn> must be treated as a shorthand for the 'column-gap' property
+	* <dfn property>grid-gap</dfn> must be treated as a shorthand for the 'gap' property
 
 	In all three cases, the legacy properties must take the same grammar as the property they are aliasing,
 	and just "forward" the value to their aliased property.