[css-grid] Add issue about grid-row-gap vs row-gap

Author: fantasai

css-grid/Overview.bs

@@ -2880,6 +2880,11 @@ Gutters: the 'column-gap', 'row-gap', and 'grid-gap' properties</h3>
 
 	On <a>grid containers</a>, a value of <dfn for="row-gap,column-gap" dfn-type=value>normal</dfn> computes to zero.
 
+	Issue: Should we have grid-specific properties for this (e.g. 'grid-row-gap')
+	instead of sharing with multicol ('row-gap')?
+	Sharing with multi-col could be problematic if we re-use the 'grid-template'
+	properties to set up <a href="http://www.w3.org/TR/css-line-grid-1/">grid-snapping</a>.
+
 	<pre class='propdef'>
 	Name: grid-gap
 	Value: <<'column-gap'>> <<'row-gap'>>?