[css-grid-3] Add 'masonry' shorthand.

Author: tabatkins

css-grid-3/Overview.bs

@@ -396,8 +396,28 @@ Masonry Flow Directions: the 'masonry-flow' shorthand</h4>
 
 	The 'masonry-flow' property is a [=shorthand=]
 	for the 'masonry-direction' and 'masonry-fill' properties.
+
+
+<h4 id=masonry-shorthand>
+Masonry Definition Shorthand: the 'masonry' property</h4>
+
+<pre class="propdef shorthand">
+Name: masonry
+Value: <<'masonry-template-areas'>> || <<'masonry-template-tracks'>> || <<'masonry-direction'>> || <<'masonry-fill'>>
+</pre>
+
+The 'masonry' shorthand sets
+'masonry-template-areas',
+'masonry-template-tracks',
+'masonry-direction',
+and 'masonry-fill' at the same time.
+
 </section>
 
+
+
+
+
 <h2 id="masonry-track-templates">
 Masonry Track Specification</h3>